I have updated your fiddle
Fiddle Link:- Click Here
Send your image to back using canvas.sendToBack(imgX);
fabric.Image.fromURL('http://mozorg.cdn.mozilla.net/media/img/products/badge-firefoxos.jpg?2013-06', function(img) {
var imgX = img.set({
top: 0,
left: 0,
});
canvas.add(imgX);
canvas.sendToBack(imgX);
});