Here is what we do in the Sempervivum Forum. The thread "Sempervivum 'Strider'" in Sempervivum forum
If it is just a single photo I try to contact the owner of the photo either with a comment to the photo or by treemail. After discussion and agreement the photo is moved to the generic Sempervivum entry. That way the photos don't disappear from any of the threads it may have been posted to.
