In my opinion you don't need AJAX for this. If the image's URL is always the same you can just use setInterval to set it to the real URL but with a spurious querystring to force a refresh. E.g. if the URL is getMyImage.php?id=123 then set it to getMyImage.php?id=123&r=123456 where the r= part is a random number, based on the time perhaps.
Joe (Microsoft MVP - XML