Check out www.voicethread.com, it is a free site that allows you to collaborate or discuss images, documents, and videos and allows people to leave comments in 5 ways - using voice (with a mic or phone), text, audio file, or video (via a webcam).I have used it so far for introducing students to new artists and it has worked great. I would basically provide an image with a title or artist name, and a description of "Tell me anything you can about this artist or image." Something like that. I have only done it twice, but i plan on doing it every few weeks.
I think think it could also be great for student critiques.
Here is an example from my first class test.
