What you want won't work because of security reasons. What you could do, however, is adding a file input field and upload it to the canvas, like this:
HTML
<input type="file" id="file_input">
JS
$("#file_input").change(function(e){
    var URL = window.webkitURL || window.URL;
    var url = URL.createObjectURL(e.target.files[0]);
    var img = new Image();
    img.src = url;
    img.onload = function() {
            img_width = img.width;
            img_height = img.height;
            context.drawImage(img, 0, 0, img_width, img_height);
    }
});
I had the same problem as you not so long ago, and this code did the job when it comes to loading local images onto a canvas. I would however also recommend to eventually resize the image, so that it fits into the canvas. I used the following code (replace 460 with the size of your canvas).
var new_x = 0;
var new_y = 0;
if (img_width > img_height) {
    new_x = 460;
    new_y = (460*img_height)/img_width;
} 
else if (img_height > img_width) {
    new_x = (460*img_width)/img_height;
    new_y = 460;
}
else {
    new_x = 460;
    new_y = 460;
}
context.drawImage(img, 0, 0, new_x, new_y);