[verelo]

Like most forms of human behaviour, if you add enough resistance, people generally start looking for alternatives. With that in mind, I would suggest you consider a service similar to Mechanical Turk to perform random checks on images, and ban any accounts the violate your terms.

Using a service like this (perhaps not MTurk exactly, i'm not sure how they feel about this content type being reviewed on their service) you could identify how long it takes until you can trust an account, to reduce your expenses associated with the filtering of traffic.

Sadly, not this is not a free option (Probably 1c per image + MTurk commissions), but I imagine eventually you can optimize the process to a point where either you spend very little reviewing images, or you hit a critical mass of a user base that the site can moderate itself.