It is possible to make the picture impossible to copy - not sure how it is done but it is possible.
If the option was genuinely pay or dont use at all, I would expect 99% of the "illegal" photos out there would not be used at all, not that this photographer would suddenly receive £20 for every copy out there. In my experience, people who can afford to pay and like it enough to buy it aren't the ones taking it for free.