The primary issue I have with this idea is that I think a 'guild shield' will imply exclusivity. With the way that uploads are at the moment, anyone will be able to upload the shield that is used as an individual or for public use. It's not really viable to keep track of which shields 'belong' to a guild and deny further upload attempts of that shield. I'm not sure if it's worth it with people being able to upload graphics that can be used by everyone.