Looks like I'll need some help on this one Lawrence, I can't think of anything members want anymore other than being the greatest forum troll they can be and wanting money from ad sharing deals or selling signature links.
For most situations, having a text signature only is fine but there are some situations I can see where having image signatures is needed. Digital Scrapbooking usually has image signatures not just text signatures, but that is about the only niche I can see needing that.
I run a few dog related forums, and allowing members to add images of their pets is a big hit, But without the ability to set restrictions.. We have recently had to make announcements regarding or new policy of policing signatures.. Ive already had a few that were full blown 1024 x 680px images.. So we have ruled that 500wide x 150height is the max allowed (text or images) and anything over those dimensions in either direction will be removed without warning.