Just pixelart skills and that it be true pixelart that shows at least 1% of care and dedication to the piece. Your tombstone is just waiting for a couple more yes PJers votes and it'll be added. Note that sometimes we let stuff sit so new art in on the front page for a bit of exposure which is the same treatment yours will get when its added.
In case you haven't noticed the public queue is where PJers vote on pixels and sometimes its fast and sometimes slow other times its low voting. So the answer to your question is:
We add as quick as we can and try to add everything because we love y'alls and we want you showing your art to others :D