If you have your own server, I once wrote someone a very simple PHP image rotation page. Instead of returning HTML headers, it returns GIF headers and a random file from a list held internally.

Only trouble is, there are a number of forums which will not accept .php as an image extension.

image.php
Code:
<?php
  header("Content-type: image/gif");

  $number = rand(1, 3);

  $image = "avatar".$number.".gif";

  include($image);
?>
Name the avatars 'avatar1.gif' to 'avatar3.gif' and place them in the same directory as this php file. If you wanted 10 files, change the value of the random number by changing "rand(1, 3)" to "rand(1, 10)" and just name the avatars up to 'avatar10.gif'

An example is at http://www.dragonstalon.co.uk/test/image.php but it is just a simple signature.