If you select and image with jQuery and then use .width(), you'll get the images current width, even if it's been scaled (e.g. max-width: 100%;). You can access the images native width (even if it doesn't have any attributes which declare it) like this:
// Get on screen image
var screenImage = $("#image");
// Create new offscreen image to test
var theImage = new Image();
theImage.src = screenImage.attr("src");
// Get accurate measurements from that.
var imageWidth = theImage.width;
var imageHeight = theImage.height;
*May or may not contain any actual "CSS" or "Tricks".