I've reviewed some questions and answers here and still cannot find my problem.
What I'm trying to do is: Toggle the display property of these three images on or off depending on the size of the screen. Only one image should display at a time.
I have not applied any css to .imagephone except what listed in the @media queries section.
.imgcontainer{
    background-color: red;
    width: 100%;
    overflow: hidden;
    max-width: 1500px;
    margin: auto;
}
.imagetablet{
    display: none;
}
.imagedesktop{
    display: none;
}
/**************@Media Tablets****************/
    
    @media screen and (max-width="800px"){
        .imagephone{display:none;}
        .imagetablet{display:block;}
        .imagedesktop{display:none;}
        }
/**************@Media Desktop*****************/
    
    @media screen and (max-width="1200px"){
        .imagephone{display:none;}
        .imagetablet{display:none;}
        .imagedesktop{display:block;}
        }
<div class="imgcontainer">
        <img class="imagephone" src="BannerPicSmall.png">
        <img class="imagetablet" src="BannerPicMedium.png">
        <img class="imagedesktop" src="BannerPicLarge.png">
    </div>