use browsercache for gallery covers

apps/gallery/ajax/getCovers.php

@@ -61,6 +61,12 @@ while (($i = $result->fetchRow()) && $counter < $numOfItems) {
 
 header('Content-Type: image/png');
 
+$offset = 3600 * 24;
+// calc the string in GMT not localtime and add the offset
+header("Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . " GMT");
+header('Cache-Control: max-age=3600, must-revalidate');
+header('Pragma: public');
+
 imagepng($targetImg);
 imagedestroy($targetImg);
 ?>