Hi,
The two I use are stockimo - who seem to reject just about everything I send them, and twenty20 - who seem to accept anything and everything.
With regards to stockimo; I've kind of given up on them. I submitted over 60 images I have active elsewhere, and they approved three (yes 3!!) of them. From what I've read, they don't really appreciate quality, moreover images that have been filtered to hell and back with as many mobile imaging apps as possible!
Twenty20 - 100+ active Images for two months - 1 sale = $2.
Hope this helps.