Photo sharing can be tough to bring into the classroom because students might be tempted to post innapropriate stuff.  

Photos posted at pics4learning are screened for content but there aren't as many photos and you can't edit the phots with information or tags so it's not a true web 2.0 site.

Photos posted at Flickr are not screened but it is a true web 2.0 site with tags and other features.

An explenation of Flickr

A tutorial for Flickr

Good article on using photosharing in the classroom.

If you look up photo sharing online you might come across this wikipedia article about metadata in the form of pictures and how we use them socially.

metadata.pdf