Perhaps I am naive, but I would have though that, by definition, if an image is for purely user-based purposes, it is a user image. Regardless of whether it is a KH-related image, and regardless of whether it's derived from an official image, if it isn't uploaded for the purpose of being used on the wiki's mainspace, projectspace or galleryspace – are there any other "proper" namespaces? If so, those too – it's a user image.
Questionable comparison to date-rape aside, I agree completely with SilverCrono's point above. The raison d'etre of the wiki is to be a wiki. Talk bubbles are a pretty bauble, a decorative piece of code, but underneath that they, like the wiki, have a function, which is to show who is saying what. While there is some scope for creativity, it must not be at the cost of the function, i.e. there is no excuse for it being unclear whose bubble it is.
If you want to do something purely for the sake of it being creative, I direct you to DeviantArt, FanFiction.net, or any of a billion other sites that exist for the purpose of creativity. In the meantime, it has no place here if it's going to interfere with the actual purposes of the talk bubble.
Incidentally, the same is true of signatures, but let's not wade into that issue right now.
EDIT CONFLICT:
Okay, TDM, you officially have nothing useful to contribute to this discussion.
- ...necessary so that people they can be stored and used by new users in the future.
What? No, sorry, that's pure BS. They are in no way necessary, and the wiki is not a hosting site. Y'know, because it's a wiki.
- There is no harm in having fan art on talk bubbles...
Indeed, but that doesn't mean they should be hosted on the wiki.
- ...are you angry about people having more than one template page or that there are too many talk bubbles?