Pixlr Editor is a Chrome App that most of us are familiar with. It offers a cloud based image editor with features similar to Gimp or Photoshop.
The biggest advantage is it can be run as a Chrome App or directly from the website. The latter could be beneficial for students without Gmail accounts.
As you can see in the video the one area that may be a challenge is the use of layers. While layers are definately useful for more advanced image editing, it was not clear to me why I needed to add a layer to simply use another tool. I could see this being a challenge for younger students.
* If you are having difficulty accessing Pixlr, try clearing your cookies in Chrome or accessing it in Incognito.