What HTTP caching headers will prevent iPhoto from re-downloading each image in a "Photo Feed" every time it checks the feed for new content?

Question

I've noticed a practical issue with iPhoto's File->"Subscribe to Photo Feed..." feature.
Every time that iPhoto checks a subscribed RSS Photo Feed for new content it will re-download each and every photo in the feed.  This occurs even if iPhoto has already downloaded each image, they all have unique GUIDs set, and no content in the feed has changed since the last time it polled for it.
My best guess is that this happens if the server offering the feed does not produce the proper cache-control parameters in the HTTP Headers that are sent by the server when the RSS feed is accessed by iPhoto.
Does anyone know what parameter/value pairs need to be set in the HTTP headers to prevent iPhoto from re-downloading RSS enclosures that it already has?
This is a very practical problem for feeds with a large number of photos of large file size.  Besides the obvious massive waste of bandwidth, the user receives an annoying error whenever they try to quit iPhoto before the feed and all of it's images have been re-downloaded yet again.

Answer 1

Here is an example of the HTTP headers for two of the images in the feed.
Any red flags in there that you think might be causing iPhoto to think it needs to re-download the images after it's already downloaded them once?
HTTP/1.1 200 OK
Content-Length: 1251893
Date: Fri, 27 Jan 2012 15:37:49 GMT
Server: Apache
Set-Cookie: PHPSESSID=1b9d47ae170ba8c0a088ab7124a1677e; path=/
Expires: Mon, 24 Jan 2022 15:37:49 GMT
Cache-Control: max-age=315360000,public
Pragma: public
Last-Modified: Thu, 26 Jan 2012 23:34:38 GMT
Accept-Ranges: none
Connection: close
Content-Type: image/jpeg;
HTTP/1.1 200 OK
Content-Length: 744151
Date: Fri, 27 Jan 2012 15:39:01 GMT
Server: Apache
Set-Cookie: PHPSESSID=086c112f99ccecc266a47d66d6b47733; path=/
Expires: Mon, 24 Jan 2022 15:39:01 GMT
Cache-Control: max-age=315360000,public
Pragma: public
Last-Modified: Thu, 26 Jan 2012 02:54:40 GMT
Accept-Ranges: none
Connection: close
Content-Type: image/jpeg;