A couple of simply rules.

1. If it is not your own fan art, please give credit to the artist unless you could not find it.

2. Please criticize openly and honestly. It'll help that artist get better as an artist.

3. Don't do anything obviously foolish, like posting an actual picture from the series that PeachPit owns. Doing that is likely to get the thread deleted and if possibly a banning from the guild.