Actually, it
is a feature, not a bug. In the UBB control panel, you can disable images in signatures. This is easily circumvented if the UBB still allows images in posts though, because people can just copy-and-paste sig code into a post, or just edit the post.
HTC, please enable pics in sigs. It's in the general settings area of the UBB control panel, but then you prolly know that.