I'd really like to use other people's photos in my synths, I think this would make Photosynth much more powerful.
The easiest way to do this would be to choose the map location and approximate area of the synth before creating it, then allow the user to choose photos of a compatible license which match this location. Order the photos by the number of synths they appear in, so such photos become gateways into lots of collections.
By allowing users to also use arial photography from Microsoft Maps and Virtual Earth, we could begin painting the planet with 3D imagary!
It would also be nice to have the ability to adjust the orientation of the point cloud so that it matches the maps, but that's a different topic altogether.