RealtimeBoard is an online whiteboard that is a good tool for real-time collaboration. It’s easy to use, and lets you upload images from your computer or by its url address.
It’s on The Best Web 2.0 Applications For Education In 2013 – So Far list, as well as being on The Best Online Virtual “Corkboards” (or “Bulletin Boards”) list.
They’ve had a limited free plan for everybody, but they just announced a free “Pro” account for educators. It’s easy to register for it here.
Here’s a video about the site and its features: