moving

Hey all... I have a site where people have to pay for the pictures. He is a professional photographer and does not want people stealing his work. When they download and image, they pay for it. The problem with that is, if I have images named 001.jpg, 002.jpg, people can type that into the URL and get the other image without paying for it. Is there a way to rename a file once clicked on to download to eliminate this from happening? So, the user clicks on the image, called from the database, have it renamed, and then delete it from the database, so they never see the real filename... I have thought of other ways to do this, but I was hoping there would be an easier way, such as this, to do it. Thanks for the help! Bryan Fitch