Flickr that I just learned about in the Images4Education course of EVO. I had never noticed before that above many photos is a feature called Add Note. After you click on it, you can click on whatever item in the picture you want and add a note (a label, description, etc.).
This could be an excellent feature to use for second language teaching. I've posted an example here called Jane Goodall's Camp. The notes will not show up here in my blog, but if you click on the picture, you will see the photo with all its notes.
It seems to me that only the owner of the image can add notes, but I'm not sure about that.