I very much support this kind of movement!
If I were to decide, there would be no signatures at all
You are going into the right direction. All it takes is a quick look into the official forums to see what kind of mess can be created with loose rules about signatures...
Makes the job for moderators a bit easier. You could even make an automated check so that the website would simply reject images in signatures, requiring no actions from moderators.
Baddo.