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!