Assuming that the new image would show when the next person gets to the specific page (or pages), then you probably need Cron or something like that to trigger a (PHP) program that would set an indicator / name for which image to use. The indicator would have to be in your database so all subsequent sessions are able to check it.
This, of course, assumes that the image to be used and the time to change from one to the other isn't predictable. If it is, then you would just put the code into the program that displays the image, check the time and then display the appropriate image.