If you don't want visitors to manually refresh and see a suspiciously different picture of you almost instantly, I suggest you use something like this (assuming you're running PHP).
PHP Code:
<?php
srand(floor(time()/30));
// seed uniquely only once every 30 seconds
// load a list of all the images
$imagesArray = glob('/pathtoimages/*.jpg');
if(count($imagesArray) > 0){
header('Content-type: image/jpeg');
// output a random image
readfile($imagesArray[array_rand($imagesArray)]);
}
?>
Incidently, I haven't tested that at all.