Pretty much what happens now--name and shame, get the story out there. If McDonald's wanted to plaster a billboard with someone's personal family photos, the odds that that family could even afford a lawyer for recourse to an appropriate degree is essentially nil. What would likely happen is that McDonald's would settle for some absurdly low dollar value and perhaps take down the billboards--or just as likely, negotiate for use in the settlement agreement, saying "take this and let us use the photo or we'll see you in court."

If someone gets a reputation for stealing others' work continuously, who is ever going to work with them?

copyleft. make contributions voluntary, credit mandatory, and commercialization impossible. gift your creations to the collective knowledge of humanity, and if people like it, they will in turn give you support. cut out corporate middle men, and cultivate an audience that will reward you generously for what you give to them.