I wouldn't try to sell them - there's a lot of supply in that market right now. But people will always visit free once they know about it - it's then your job to figure out how to monetize them while keeping the nuisance factor for the user to a minimum.

That said, my site seems to get picked up in an unhealthy number of "top free images for your website" type list articles - I'm not sure if other sites would get picked up at the same rate, but you could always try.