When I worked for a porn web site, what they did for the sample front page images was to have a web page refer to an image, "pic1.jpg". Then once a day, a script would randomly select a picture from a gallery, and copy or link that to be accessed from "pic1.jpg".

You could also save the generated value in a file or in a database. That might be more appropriate if it isn't simply one of a numbe of files, but something more sophisticated.