When you sell stock photography, there is always a risk that someone may use your image in a way that you didn't permit in the licensing agreement. When that happens, you need to protect your licensing rights. The best way to do that is to work with a copyright and licensing attorney. I've spent a lot of time researching fair use and licensing restrictions. I hope that the information here helps you to not only understand your rights as the creator but also to learn how to document and enforce those rights and seek legal resolutions when they are violated.