btw the other suggestion prolly wont work. Reason is it outputs html whereas using the image function outputs actual image header info for png, jpg, gif or whatever type you tell the php code to dynamically generate.
take this sample code from the php.net link I gave before...
Code:
<?php
header("Content-type: image/png");
$string = $_GET['text'];
$im = imagecreatefrompng("images/button1.png");
$orange = imagecolorallocate($im, 220, 210, 60);
$px = (imagesx($im) - 7.5 * strlen($string)) / 2;
imagestring($im, 3, $px, 9, $string, $orange);
imagepng($im);
imagedestroy($im);
?>
Now modify that with some random number generation to do a
if($num == 1) {$im = imagecreatefrompng("pic1.png");
if)$num == 2) ($im = imagecreatefrompng("pic2.png");
etc...