I'm fairly sensitive to this issue because I have seen my work used to make others money without my permission. I once was at an amusement park and saw some one wearing a T-shirt that had one of my images on it. I also did a Google search of my name and found some one selling prints of my work. I tracked it down and some guy that once worked at a service bureau that I had worked with had stolen a large amount of files.
Last year, a student came to me and told me that a bike shop across the country was using a logo that she created. They took it right off her web-site. When we contacted the shop, they were horrified. When they got back to us, they told us that the graphic designer that they had hired admitted to stealing the image. The Bike shop ended up paying my student market value for the image. They were in the process of trying to get their money back from that lazy designer.