Two ways I can think of. [ed]ok, three[/ed]
If it is a remote site then you have to do it the hard way by programming the client to do it, use an array (Javascript/VBS) and randomize the order of the links to the images
OR
If it is a local site and you have control on the server, the easy way is to use static references in the page, but run a script on the server to randomly change the file name of the images to match your static references. This way you can cycle through a much larger number of images but only show a smaller number on the page like web ads, simple but effective.
OR
You can even implement this on a remote site by scripting the filename change locally and then re-sending the files up to the remote site periodically.