Run this script as a bookmarklet or in your console to detect if you have oversized background images on your page

backgroundimagesize.js

(function() {

    // the minimum version of jQuery we want
    var v = "1.3.2";

    // check prior inclusion and version
    if (window.jQuery === undefined || window.jQuery.fn.jquery < v) {
        var done = false;
        var script = document.createElement("script");
        script.src = "http://ajax.googleapis.com/ajax/libs/jquery/" + v + "/jquery.min.js";
        script.onload = script.onreadystatechange = function() {
            if (!done && (!this.readyState || this.readyState == "loaded" || this.readyState == "complete")) {
                done = true;
                initMyBookmarklet();
            }
        };
        document.getElementsByTagName("head")[0].appendChild(script);
    } else {
        initMyBookmarklet();
    }

    function initMyBookmarklet() {
        (window.myBookmarklet = function() {
            jQuery('*').filter(function() {
                if (this.currentStyle)
                    return this.currentStyle['backgroundImage'] !== 'none';
                else if (window.getComputedStyle)
                    return document.defaultView.getComputedStyle(this, null)
                        .getPropertyValue('background-image') !== 'none';
            }).each(function() {
                var imageSrc = this.style
                    .backgroundImage
                    .replace(/url\((['"])?(.*?)\1\)/gi, '$2')
                    .split(',')[0];
                var image = new Image();
                image.src = imageSrc;

                var width = image.width,
                    height = image.height;

                if (width != 0 && height != 0) {
                    // calculate percentages
                    var pctw = Math.ceil(this.clientWidth / width * 100);
                    var pcth = Math.ceil(this.clientHeight / height * 100);
                    console.log(imageSrc + " " + pctw + "% x " + pcth + "%");
                }
            });
        })();
    }

})();