Poodll and me got mentioned. So I thought I would speak up.
To the original question:
Atto_sketch looks good, and it uses LiterallyCanvas which we also use in Poodll. So try that out. Be sure to choose "html editor with file picker" in the response type, so that it will accept a file upload (the whiteboard image).
About Poodll and drawing.
Poodll does support drawing and uses Literally Canvas OR DrawingBoard.js. Both support saving the vector data along with the raster data. That means that the student can continue editing where they left off, when they return to the question or assignment later. ie they can use the back button etc. And they support annotating over a background image.
About Poodll and going commercial.
I definitely see the anomaly of commercial plugins amongst free moodle and open communities like Moodle.org. Poodll went commercial because it was very time consuming, though I loved it. At a particular crossroads I had to answer the question: "Is this a hobby, or a profession?"
Moodle had gone from 1.9 to 2.x to 3.x. With lots of rewrites and late nights. And then along came smart phones and Steve Jobs decided to kill Flash, and I had to decide ...do i rewrite mostly everything to support html5, or walk away. I tested the waters and took pre-orders for Poodll3 and a good handful of Moodlers came through. And that is how it went.
Poodll and Poodll users have benefited massively from going commercial. Its never been more reliable or supported so many platforms. And the improvements are ongoing. We are working hard to make it fully accessible and to support various data privacy laws etc. And I get to do what I love full time.
But its not the road for every plugin or developer, I know that.