Getty Images actually pays a company that runs bots crawling for their images. If liscences don't match up they will send you an "invoice" in the mail much like the RIAA does for file sharers.
In order to register the copyright of an image the photographer must pay $30 for every picture. Do the math, very few can afford it. Without the copyright registration they have to prove actual damage and a lot of stuff. Put it this way, its the difference between $1,000 and $150,000 from an actual infrindgement.
In the US we have the DMCA. Basically if your users are submitting copyrighted work they get in trouble, not you. This is why sites like Youtube are alive and well.
From what I have observed there are specific companies and individuals who will agressively pursue action against those that infrindge on their copyright. Quite honestly, you could lose your site, house, etc. Unfortunately there are also very grey fair use areas that the DMCA and other different laws create making this a real minefield..