To gain publicity, I did a shoot for an independent clothing label. I told him that he could have 10 of the photos for free, and if he wanted more, he would have to pay for additional ones at a flat rate. He received every photo as a tiny jpeg with a my copyright on it.
After sending the photos, he asked for 30. I suggested a cost that was less than the quoted flat rate for the additional 20. He he refused to pay me.
Two weeks later, I discover he used the tiny preview jpegs on his website to advertise his products. My (large) copyright is still visible on each and every one.