Well, if you have explicitly defined the width and the height of the image through CSS, you can get it like this:
var width = document.getElementById("myImage").style.width;
var height = document.getElementById("myImage").style.height;
...of course, I think you said you didn't specify with CSS. If that's the case, I think the only solution is to specify it...heh...*looks around* I sure hope someone has a better solution that that.