ok very bad idea- im not actually getting any images, but i'll keep re-guessing. seems to work best if the first digit is '2xxxxx'
The most recently uploaded image on the site is #286040, so I'm pretty sure that means that is the highest number currently in use. Some of the later numbers would also have been assigned to entries currently in challenges, so you'll get an error about "this image being in submission phase" for those. And some numbers just don't exist, perticularly in the really low numbers if I remember correctly (there was discussion about this a while back...).
