Modern browsers

When I wrote this answer back in 2009 the browser landscape was much different. Nowadays any reasonably modern browser supports Pim Jager's suggestion using img.naturalWidth and img.naturalHeight. Have a look at his answer.

Legacy answer compatible with super old browsers

// find the element

var img = $('#imageid');



/* 

 * create an offscreen image that isn't scaled

 * but contains the same image.

 * Because it's cached it should be instantly here.

 */



var theImage = new Image();

theImage.src = img.attr(src);



// you should check here if the image has finished loading

// this can be done with theImage.complete



alert(Width:  + theImage.width);

alert(Height:  + theImage.height);