This is one of those "can't please all people all the time" kind of things. If we allowed images in signatures then a bunch would complain that the images in signatures are bothering them.
We decided early on that we wanted to keep the forum reading experience to be as fast and clean as possible. We've analyzed other forums where images are allowed in sig lines and either:
1) People abuse it and put huge images that take forever to download (especially a problem because we have a lot of members on dialup) and put huge images between posts ever time that person posts a reply
2) It becomes a nightmare for the staff to police which signature images are and aren't a problem
There have been many suggestions by members and even some of the staff to remove signatures completely. We decided that allowing signatures without images was a great compromise.
Remember, everything we do is with the goal of making the community better, stronger, and easier to access for everyone around the world!