Personally, I'd highly recommend a license grant for ANYTHING uploaded to your server. Otherwise you can get in to odd legal situations where you are providing content that you don't have the right to present if they decide to be a pain. If someone want's to post content that they do not wish to release, they can post it to their own site and cross-link it.

You can then use a typical agreement like CC-WIKI for covering anything posted to the board, however users still have an option, at their own expense, to provide links to content (or even embed it) which they are not contributing. It is then up to them to manage their own content and you can keep anything posted on the forum.

This also doesn't mean that they are giving up rights to their images or content. They are still the copyright holder and can use it for any purpose they want. All it means is that in sharing, they agree to actually share the content if they are posting it to your server. Ie, you are allowed to post it. You can alternately use other license terms if you think CC-WIKI is too broad, but I'd stick to the my server/there server distinction for clarity. At a minimum, anything posted to your server should be licensed for you to share forever.

This is actually the same structure under which StackExchange operates and it seems to have worked pretty well overall.

Personally, I'd highly recommend a license grant for anything uploaded to your server. Otherwise you can get in to odd legal situations where you are providing content that you don't have the right to present if they decide to be a pain. If someone wants to post content that they do not wish to release, they can post it to their own site and cross-link it.

You can then use a typical agreement like CC BY-SA (a.k.a. CC Wiki) for covering anything posted to the board; however users still have an option, at their own expense, to provide links to content (or even embed it) which they are not contributing. It is then up to them to manage their own content and you can keep anything posted on the forum.

This also doesn't mean that they are giving up rights to their images or content. They are still the copyright holder and can use it for any purpose they want. All it means is that in sharing, they agree to actually share the content if they are posting it to your server. I.e., you are allowed to post it. You can alternately use other license terms if you think CC Wiki is too broad, but I'd stick to the my server/their server distinction for clarity. At a minimum, anything posted to your server should be licensed for you to share forever.

This is actually the same structure under which Stack Exchange operates, and it seems to have worked pretty well overall.