I'm doing some simple image preloading, but for some reason that I cannot for the life of me figure out, IE is reporting erroneous dimensions for the image:
    var prel = new Image();
    prel.onload = function() {
        alert(prel.width + ' ' + prel.height);
    prel.src = 'image.jpg';
It's part of some longer code, so I just tested the above snippet by itself with no other JS to influence it and IE is still reporting the wrong dimensions. The odd thing is it seems to do it only for some images I try and in some cases the difference from reality varies by over 100px, in others by only a few px.

I've cleared my cache approximately 1 billion times, restarted the PC and still the erroneous dimensions are being reported. Other browsers are behaving.

Any idea what it could be?