I've only experienced one situation when this was an issue for us. We were given permission by a professional photographer to use her photos. When someone requested hi-resolution copies for his marketing purposes, we told him that he would need to contact the photographer directly for a release.
We treat our photos like our other published resources--they're freely available except for commercial use (so, in our case, we would not authorize them for businesses to use). We've never--to my knowledge--actually enforced this, though.
The state of Mississippi's website, ms.gov, has a Flickr page where people can share their photos of Mississippi, which sounds a bit more like your situation. Their rules [http://www.flickr.com/groups/msgov/rules/] are pretty straightforward.
I hope this helps!