JavaScript to replace images with placeholders and display their height and width in a div layered on top.

display-image-sizes.js

(function(){
  jQuery("img").each(function() {
	var h, w, x, y, m, i;

	h = this.height;
	w = this.width;
	l = Math.ceil(jQuery(this).offset().left) + 5;
	t = Math.ceil(jQuery(this).offset().top);

	jQuery('section').css('position', 'relative');

	m = "Image Size: " + w + "x" + h;
	i = "http://placehold.it/" + w + "x" + h;

	jQuery('<div><p>'+ m +'</p></div>').css({
	    "position" : "absolute",
	    "left"     : l,
	    "top"      : t,
	    "zIndex"   : 1000
		}).appendTo(jQuery('body'));

	jQuery(this).attr('src', i);

  });
})();