The reason that images aren't allowed in signatures are NOT because of bandwidth (as you said the pictures are linked to another site where they are hosted). The reason is that people end up having all sizes of pictures and makes it hard to distinguish forum content from pictures in signatures and things get very messy.
Sure we could say that 'certain' pictures sizes are allowed, but it becomes impossible to moderate and 'measure' the size of people sig pictures.
So the easy and proffessional way to do it is to dissalow signatures and make the forum look more informative.
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