HT: Dom Latter
Some people on my local list are all of a tizz because they were getting threatening letters with £800 bills attached from Getty Images - Corbis also does this. Dom Latter reminded people that, no, Getty cannot 'name their price' (my precis of his points follows).
Because it's copyright infringement, all they can do is sue (for loss of earnings). To his knowledge - and feel free to correct me - this process has now been going on for many years without anyone actually being taken to court during this period.
As a layman, I don't see how an invoice for an abitrary amount can be made to stick in the absence of a contract; Getty are of course free to claim damages for the infringement of their copyright, but it would appear that they don't,
out-law makes it clear that they cannot sue for "additional" damages, in the case of *unwitting* infringement.
In addition, Dom provides links to cases of people who "got caught and got out of it".