Use the same expiration rules than the original server.

[guillermo]

If the original server support expiration times, copy that times in the resulting image. If the original server support e-tags, copy that e-tags from client to server.

[radiospiel]

Well, at the moment I won't. While support for these could probably be useful, I don't need them at the moment.

• ETags are of litte use here, as they would need a request to the originating server, and the idea behind this is just to not send that many requests.
• where I am using it right now, this piece must be able to work with "misconfigured" servers. I do know, that images there never change, so expiration time support wins me nothing, whether or not it is configured properly.

But if you or someone else wants to lend a hand I will happily merge it in.