No one likes to see their images used without compensation, particularly if the use is on a site that generates revenue for the user.
PicScout, a company that searches the web for images found in the collections of professional image distributor, says that 85% of the images they find on commercial sites are unauthorized uses (either never licensed, or used beyond the license). This article deals with what you should do to locate images that have been infringed and to receive compensation.