just throw a little bit of php on your page and youre good to go
<?php
$img[1] = "location/of/some/image.jpg";
$img[2] = "location/of/some/image.jpg";
$img[3] = "location/of/some/image.jpg";
$img[4] = "location/of/some/image.jpg";
$r = rand(1, count($img));
echo "<img src=\"" . $img[$r] . "\" />";
?>