I'm also pretty sure that it does resize certain things. A member or two reported back a few months ago that they were trying to upload a GIF but because it either exceeded (a) file size or (b) dimensions it would reduce either or and then the GIF would be static and not have the animation.
Besides, why would you want the forum to auto-resize your images anyways? No forum plugin would be advanced enough to resize the image and maintain the proportions of the image which results in stretched images if the ratio isn't there. Thus, if you wanted the forum to resize it appropriately you'd still have to crop the image so that it can reduce to the maximum dimensions your user group has. Does that make sense?