It makes sense why non-official images would be kept out of the general wiki. But why is there no category for User Images like there is on the Fairly Oddparents wiki, SpongeBob wiki, Sonic wiki and others?

I say we add one. Images with this category would be used exclusively in ones' userspace and wouldn't interfere with anything else. "Official only images" can be quite limiting and I think it'd be great for people to be able to customize their user page how they like. (obviously, inappropriate images wouldn't be allowed)