Nice job guys. This is a very fun idea.
Just a thought, but depending on how rasteresque you need the drawings to look, you might find it easier, more flexible and work better with the infrastructure in production to use svg paths instead of canvas.
You’d be able to post as character data and even if the paths were drawn on top of a raster image you could add layers so that multiple people might be able to collaborate on a drawing or whiteboard annotation in some sort of pseudo realtime which would be much more difficult and take an absurd amount of bandwidth with raster data and file uploads/downloads.