The 'best' way depends on what resources you have to deal with. Personally, I'd just set a cron job to randomly select the image each morning, and move it from whatever holding spot I had to where the general public would see it.

If you only have CGI, and no cron, then I'd just use a randomizing function that took the time as a seed, and was deterministic. For instance, the modulus of number of days since Jan 1, 2000 and the number of images I had to rotate, if it were to change on a daily basis. (no, it's not completely random...it's just an example ... you can use whatever function you want, so long as it returns the same value for each seed.) If you want it more random, just use rand() and Memoize.