I don't pay any attention to signature images at all. Except that sometimes they make it more challenging to read the forum because of such profuse eye candy. It's too easy to end up with people having similar images so it makes more sense to identify their posts by their names. Hence, images are in most cases just individual visual vanity fluff. If you want to allow them, that's fine. Keep them small, and off to the side so they don't create screen clutter, please.