Html5 is at this time the future of the web especially in terms of graphics. If you do a web search on html5+canvas, you will fine a boatload of information and usable snippets. Starter html5 web based drawing programs. The first one is for younger kids.
Napkin notes: (aka canvas paint)
This next program draw.io is definitely going on the server!
Free book code: http://www.html5canvastutorials.com/cookbook/
$ wget -B http://www.html5canvastutorials.com/cookbook/ -r http://www.html5canvastutorials.com/cookbook/
Online tutorials: http://www.html5canvastutorials.com/tutorials/html5-canvas-element/
Free book teasers: