[LeonidBugaev]

I've made similar prototype few years ago, if someone interested here is source code:

https://github.com/buger/PointOut

Realtime stuff build on top of GAE.

[matb33]

Built one too, but with Meteor. I actually create a login for you automatically with auto-generated username and you can rename your username if you care. Also can copy paste images, not just drag drop

http://sket.sh/