Generated images expire immediately

image.module sets the Expires header to the current time, which causes the images to be reloaded repeatedly. I don't see why it's necessary to explicitly set the Expires header, so I changed the code
to set the image's Last-Modified header instead.

--- image.module.orig   Mon Oct 18 22:15:01 2004
+++ image.module        Mon Oct 25 21:42:15 2004
@@ -982,7 +982,7 @@
 
   // Set up headers for output.
 
-  $header[] = "Expires: ". gmdate("D, d M Y H:i:s") ." GMT";
+  $header[] = 'Last-Modified: ' . gmdate('D, d M Y H:i:s', filemtime($imgname)) . ' GMT';
   $header[] = "Content-Type: image/$node->img_format";
   $header[] = "Content-Length: ". filesize($imgname);