The "preview image" seems to be a scaled-down resampling of the original to 100x100, and this won't do. You have to make sure that what you use as preview is pixel art.
It's very common to use a cropped version of the larger piece (pick an interesting square), with the word 'preview' or '+' somewhere to entice people to click it to see the full piece.

Edit: I just saw the treant has the same issue, please do the same kind of preview for it. You can edit the piece to provide the new preview, it will stay visible in your gallery without having to pass in the public queue again.
If you see it "sent back for revision", it will be because a moderator noticed the problem and manually sent it back. For any "revision needed" pixel, you just have to update the image and then hit the submit button.


Edited by yrizoud - 22 December 2014 at 2:46pm

^ see my edit above also.