I spent who knows how long trying to research this beforehand, yet I have found no answer suited towards my problem.
Currently, I am creating 22 Rectangle Objects, in which I want to assign an image to each of them, which is the back of a playing card. However, when I try to, the image of a playing card appears for a split second, before the image reverts to a black rectangle.
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
import me.potato.applications.enums.Card;
import java.awt.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Random;
public class Main extends Application {
    private static int width;
    private static int height;
    private static Button hitButton;
    private static Button revealCardsButton;
    private static Random r;
    private static ArrayList<Card> user;
    private static ArrayList<Card> dealer;
    private static ArrayList<Card> cards;
    private static Rectangle U0,U1,U2,U3,U4,U5,U6,U7,U8,U9,U10,D0,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10;
    public static void main(String[] args) {
        width = ((int) Toolkit.getDefaultToolkit().getScreenSize().getWidth());
        height = ((int) Toolkit.getDefaultToolkit().getScreenSize().getHeight());
        user = new ArrayList<>();
        dealer = new ArrayList<>();
        cards = new ArrayList<>();
        cards.addAll(Arrays.asList(Card.values()));
        r = new Random();
        U0  = new Rectangle(width / 12, height / 4.5);
        U1  = new Rectangle(width / 12, height / 4.5);
        U2  = new Rectangle(width / 12, height / 4.5);
        U3  = new Rectangle(width / 12, height / 4.5);
        U4  = new Rectangle(width / 12, height / 4.5);
        U5  = new Rectangle(width / 12, height / 4.5);
        U6  = new Rectangle(width / 12, height / 4.5);
        U7  = new Rectangle(width / 12, height / 4.5);
        U8  = new Rectangle(width / 12, height / 4.5);
        U9  = new Rectangle(width / 12, height / 4.5);
        U10 = new Rectangle(width / 12, height / 4.5);
        D0  = new Rectangle(width / 12, height / 4.5);
        D1  = new Rectangle(width / 12, height / 4.5);
        D2  = new Rectangle(width / 12, height / 4.5);
        D3  = new Rectangle(width / 12, height / 4.5);
        D4  = new Rectangle(width / 12, height / 4.5);
        D5  = new Rectangle(width / 12, height / 4.5);
        D6  = new Rectangle(width / 12, height / 4.5);
        D7  = new Rectangle(width / 12, height / 4.5);
        D8  = new Rectangle(width / 12, height / 4.5);
        D9  = new Rectangle(width / 12, height / 4.5);
        D10 = new Rectangle(width / 12, height / 4.5);
        U0.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U1.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U2.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U3.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U4.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U5.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U6.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U7.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U8.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U9.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U10.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D0.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D1.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D2.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D3.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D4.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D5.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D6.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D7.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D8.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D9.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D10.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        launch(args);
    }
    @Override
    public void start(Stage stage) throws Exception {
        hitButton = new Button("HIT");
        hitButton.alignmentProperty().setValue(Pos.CENTER);
        revealCardsButton = new Button("REVEAL CARDS");
        revealCardsButton.alignmentProperty().setValue(Pos.CENTER);
        VBox root = new VBox(5);
        root.setAlignment(Pos.CENTER);
        root.setPrefSize(width, height);
        HBox cardSetDealer = new HBox(5);
        HBox hitButtonContainer = new HBox(5);
        HBox cardSetUser = new HBox(5);
        HBox revealCardsButtonContainer = new HBox(5);
        cardSetDealer.setAlignment(Pos.CENTER);
        cardSetUser.setAlignment(Pos.CENTER);
        cardSetDealer.getChildren().addAll(D0,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10);
        cardSetUser.getChildren().addAll(U0,U1,U2,U3,U4,U5,U6,U7,U8,U9,U10);
        hitButtonContainer.getChildren().add(hitButton);
        revealCardsButtonContainer.getChildren().add(revealCardsButton);
        root.getChildren().add(cardSetDealer);
        root.getChildren().add(hitButtonContainer);
        root.getChildren().add(cardSetUser);
        root.getChildren().add(revealCardsButtonContainer);
        Scene scene = new Scene(root);
        stage = new Stage();
        stage.setScene(scene);
        stage.show();
    }
}
I am unsure what I am doing wrong, or if I am misusing the .css segment, if my code itself is faulty, if I'm just stupid, etc.
--EDIT--------
Changed code due to @James_D 's suggestion to use Region over Rectangle.
import javafx.application.Application;
import javafx.css.Style;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.Background;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Region;
import javafx.scene.layout.VBox;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
import me.potato.applications.enums.Card;
import java.awt.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Random;
public class Main extends Application {
    private static int width;
    private static int height;
    private static Button hitButton;
    private static Button revealCardsButton;
    private static Random r;
    private static ArrayList<Card> user;
    private static ArrayList<Card> dealer;
    private static ArrayList<Card> cards;
    private static Region U0,U1,U2,U3,U4,U5,U6,U7,U8,U9,U10,D0,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10;
    public static void main(String[] args) {
        launch(args);
    }
    @Override
    public void start(Stage stage) throws Exception {
        width = ((int) Toolkit.getDefaultToolkit().getScreenSize().getWidth());
        height = ((int) Toolkit.getDefaultToolkit().getScreenSize().getHeight());
        user = new ArrayList<>();
        dealer = new ArrayList<>();
        cards = new ArrayList<>();
        cards.addAll(Arrays.asList(Card.values()));
        r = new Random();
        U0  = new Region(); U0.setPrefSize(width / 12, height / 4.5);
        U1  = new Region(); U1.setPrefSize(width / 12, height / 4.5);
        U2  = new Region(); U2.setPrefSize(width / 12, height / 4.5);
        U3  = new Region(); U3.setPrefSize(width / 12, height / 4.5);
        U4  = new Region(); U4.setPrefSize(width / 12, height / 4.5);
        U5  = new Region(); U5.setPrefSize(width / 12, height / 4.5);
        U6  = new Region(); U6.setPrefSize(width / 12, height / 4.5);
        U7  = new Region(); U7.setPrefSize(width / 12, height / 4.5);
        U8  = new Region(); U8.setPrefSize(width / 12, height / 4.5);
        U9  = new Region(); U9.setPrefSize(width / 12, height / 4.5);
        U10 = new Region(); U10.setPrefSize(width / 12, height / 4.5);
        D0  = new Region(); D0.setPrefSize(width / 12, height / 4.5);
        D1  = new Region(); D1.setPrefSize(width / 12, height / 4.5);
        D2  = new Region(); D2.setPrefSize(width / 12, height / 4.5);
        D3  = new Region(); D3.setPrefSize(width / 12, height / 4.5);
        D4  = new Region(); D4.setPrefSize(width / 12, height / 4.5);
        D5  = new Region(); D5.setPrefSize(width / 12, height / 4.5);
        D6  = new Region(); D6.setPrefSize(width / 12, height / 4.5);
        D7  = new Region(); D7.setPrefSize(width / 12, height / 4.5);
        D8  = new Region(); D8.setPrefSize(width / 12, height / 4.5);
        D9  = new Region(); D9.setPrefSize(width / 12, height / 4.5);
        D10 = new Region(); D10.setPrefSize(width / 12, height / 4.5);
        U0.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U1.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U2.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U3.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U4.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U5.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U6.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U7.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U8.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U9.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        U10.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D0.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D1.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D2.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D3.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D4.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D5.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D6.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D7.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D8.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D9.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        D10.setStyle(" -fx-background-image: url(\"..\\..\\..\\resources\\images\\cards\\BACK.png\");");
        hitButton = new Button("HIT");
        hitButton.alignmentProperty().setValue(Pos.CENTER);
        revealCardsButton = new Button("REVEAL CARDS");
        revealCardsButton.alignmentProperty().setValue(Pos.CENTER);
        VBox root = new VBox(5);
        root.setAlignment(Pos.CENTER);
        root.setPrefSize(width, height);
        HBox cardSetDealer = new HBox(5);
        HBox hitButtonContainer = new HBox(5);
        HBox cardSetUser = new HBox(5);
        HBox revealCardsButtonContainer = new HBox(5);
        cardSetDealer.setAlignment(Pos.CENTER);
        cardSetUser.setAlignment(Pos.CENTER);
        cardSetDealer.getChildren().addAll(D0,D1,D2,D3,D4,D5,D6,D7,D8,D9,D10);
        cardSetUser.getChildren().addAll(U0,U1,U2,U3,U4,U5,U6,U7,U8,U9,U10);
        hitButtonContainer.getChildren().add(hitButton);
        revealCardsButtonContainer.getChildren().add(revealCardsButton);
        root.getChildren().add(cardSetDealer);
        root.getChildren().add(hitButtonContainer);
        root.getChildren().add(cardSetUser);
        root.getChildren().add(revealCardsButtonContainer);
        Scene scene = new Scene(root);
        stage = new Stage();
        stage.setScene(scene);
        stage.show();
    }
}
 
    