Put this headers in your script because Firefox has a 'good' habit to
cache images that sometimes does this thing...
$expires = 1;
$exp_gmt = gmdate("D, d M Y H:i:s", time())." GMT";
$mod_gmt = gmdate("D, d M Y H:i:s", time())." GMT";
header("Content-type: image/gif");
header("Expires: {$exp_gmt}");
header("Last-Modified: {$mod_gmt}");
header("Cache-Control: public, max-age={$expires}");
Hope it helps,k it's working perfectly for me.
