In December '08 I did a post on EtherPad, which can be used to collaborate with other writers over the web. Now, there is the Rate My Drawings site that has a similar "DrawChat" facility where you can collaboratively view and edit drawings while chatting. You can do this in a public or private web chat room.

This web site is primarily a critiquing site for artists and you have to be a member and have "contributed" at least one drawing to use the chat facility. You'll have to investigate the artist's rights issues before using it.