For a lot of teachers, the first issue with using Flickr as a classroom resource will be convincing their districts not to have Flickr blocked by their filtering sevice. For that matter, many districts are blocking NING, Blogger and Twitter as well!
I have used Flickr for PhotoStory assignements such as:
1. Compare Past, Present and Celebration pictures of American Indians. Sound effects tribal songs.
2. Alternate images with the text of the Pledge of Allegiance to show Patriotism on Veteran's Day. Sound effects Star Spangled Banner
3. Compile Christmas images and music.
I always preselect my images, because I teach the little ones 2nd-4th. I put 40-50 images in a folder. The assignment usually involves working in pairs and the students selecting 10 pictures each. I also have a seperate folder with sound so they have choices for that.
I think using Flickr for teaching other languages is very useful. You can post a picture and have students create notes that are linked to certain parts of the image giving the vocab word in another language. Or the teacher can do that part and students can use the images for vocab practice.