PHP Code:
$color = "";
$number = rand(1,5); // Get random number between 1 and 5
if($number == 1){
$color = "redpebbleimage.gif";
}
else if($number == 2){
$color = "greenpebbleimage.gif";
}
else if(... and so on
Just do that to assign a random pebble image and save in the db. :)