This free application offers some of what I’m looking for, allowing my own geotagged Flickr photos to be easily plotted on a map.
When you click on any image in the bar at the bottom or pin in the map you get a direct link to the Flickr photo and any information given in the photo description (see below). This is a good starting point for a platform.
However, it needs to be able to pull images from a Flickr group rather than a single user and also to aggregate groups of photos from the same location/of the same sign. Also, building in some functionality like that available on this site to suggest geotags for other users’ photos would also be beneficial. Finally, the ability to allow others to add information to the photo would help with adopting the Wiki-based approach to building up research.
[Update, October 2020. Dead links have been removed from the above post.]