Google has a product in Google Docs called Drawing. You can use Google Drawing to make charts, diagrams, designs, and more. You can create everything from timelines to detailed images. With Drawing, you can insert text, shapes, arrows, scribbles or add images from your hard drive or from the Web.
This tool is a great way to work together with a class where images and drawings help with learning. Drawings can be shared with others as you draw. You can simultaneously edit drawings with others. There is a revision history so you go back and see all the edits that were made to the drawing. The drawing can be copied and pasted into another document.
Here is an example: