Here is the code for this example

<?php
//an array of 4 images in the same direcory as the script
$images = array("a1.gif","a2.gif","a3.gif","a4.gif");
//generate a random number
srand((double)microtime()*1000000);
$randomno = (rand()%4);
//display image
echo ("<img src=\"$images[$randomno]\">");
?>