I'm new to this, so if this is a stupid question, sorry: can the banner either feature original artwork or does it have to be official images?
Artwork (both fan-art and original) should be fine. It needs to be directly anime-related. If you can link to it on
AniDB it should be fine. For instance, if you made a Clannad banner, you could use art from the anime, games or other sources. If the image does not look fairly professional (like your typical fan-art), I wouldn't bet on it getting accepted, however.
@ temporalcaskNice banner, but I think Ritsu looks a bit awkward. Do the characters form separate layers? I think it looks like she's been resized wrongly, making her "wider" than she's supposed to be.