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Media Gallery is just one step behind to be usable as adserver. I have requested a feature that would allow us to specify a custom link for each individual picture, so that in a random photo block, user can click on that picture and be forwarded to the related advertiser's website.
If that feature is added, we will have a fully integrated adserver since MediaGallery is already tightly integrated with Geeklog. You might want to visit MediaGallery and cast your vote for this feature.
Sam