I don't see why not. As long as the dimensions are a fit (SD/HD & not just a transaparent bckgd image of the character). Are you asking from a marketing/allowed stance? I've never heard much otherwise, so long as they aren't distasteful. You may want to have the channel name in writing however, I'm not sure if that's required you'd need to read thru the tutorial & instructions if it's mandatory, but I would think it would be a helpful thing to do. Unless you're intentionally not doing that, again read the documentation and at worst, submit it if it works otherwise and see if it gets approved.