Any art that you have complete rights to. It can be past submissions, have been featured at another site or be for sale at another store BUT YOU MUST HAVE THE RIGHTS TO SELL IT. This is the responsibility of the artist and you legally agree to it when you upload.
Remember to keep it clean too please. Make sure your art does not contain any trademarks, logos, copyrighted material, or any other intellectual property belonging to any third party, or any material, which DBH in its sole discretion, deems to be profane or offensive.