Is there a reason why we could not use flickr and have a taging scheme to include the musicbrainz id’s?
Flickr supports tags and some of these tags have been designed to be machine readable.
There is a tagging scheme for animals where you can tag an animal by thair taxonomy to make it easier to find.
Encyclopedia of life
Having it hosted elsewhere gets around the dmca problem as they will process the take down and the commercial re-use problem as flickr allows the user to specify a licence and this may be one of the cc licence family or all rights reserved so you know you cannot use it.
I think all is required is to come up with a scheme to tag photo’s and to start writing tools that searches for these tags.
There are api’s to search flickr based on tags so it should be straight forward to integrate this type of thing.