Shutterstock is a great place to buy and sell royalty-free images!
Check it out.

Photographers:
http://submit.shutterstock.com/
Check this site out, make $0.25 per download of your images, not bad if you ask me. I signed up just a few hours ago.
Anyways, if you do decide to try it out and sign up, please go under my referal, it's much appreciated.

http://submit.shutterstock.com/?ref=100296
My referal link.

Photo Buyers:
http://www.shutterstock.com/
Same as above, but if your buying photos! Might be worth it if you are a journalist or something.
My referal link: http://www.shutterstock.com/?rid=100296

Thanks,
Ictinike