Anything that violates the terms (trademarks, copyrighted photos, other stuff) can get stuck in submitted, it has been my experience that a custom template with your own image can get stuck. A link to the terms is at the bottom of the web page.
ghostofchurch has a "imgflip 101" basics of imgflip meme
imgflip.com/i/1bc21u and alot of other good info here imgflip.com/m/community_based