Sometimes new images appear after and have a smaller number. Last example: 17452 which appeared today, after 17456 which was posted last night. In some cases, the difference can be several days. Example 15482 has still about 5 days before expiration while 15490 has expired a few days ago.
BTW why are there gaps between file numbers ? (no 17453 17454 17455)
I´m not too sure but i think i read it a while ago in the forum.
Everytime you upload a picture, it will get the the chronological next number.
Then the admins have time to vote if they should accept it or not (2 days) If the picture is disapproved it won´t be shown on wtm but the number was already given.
If not enough admins have voted to approve a picture or not the picture will be queued again and has the chance to be approved later on.
Thanks for your answer. Maybe there is something to change in this process, because if you don't scan periodically all the last images, you can miss many of them.