PHP cleverly uses 304 code to cache image files

private function _addEtag($file) {
 $last_modified_time = filemtime($file);
 $etag = md5_file($file);
 // always send headers
 header("Last-Modified: ".gmdate ("D, d M Y H:i:s", $last_modified_time)." GMT");
 header("Etag: $etag");
 // exit if not modified
 if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE ']) == $last_modified_time ||
 @trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) {
 header("HTTP/1.1 304 Not Modified");
 exit;
 }
}

Just call it before outputting static files (such as pictures).