>This is where js would be handy - right? Personally I would just update the dom element and send a http post in the background.
If it's to be done in JS, might as well implement a drawing tool in an actual canvas element.
I've seen tutorials that are reasonably simple. The complicated part would be actually saving it. It is possible to export a canvas as a png and upload it with an AJAX request but I don't know how cross-browser compatible that is.