Adding a "step-back" button (even if it was only a few states), and the ability to increase or decrease brush size would greatly enhance this. Great start though, and something like this is much needed for a lot of people who may not know an image editor, or don't want to open it for something simple.
Undo is sorely needed. Just deployed a smaller brush size (30px instead of 40px when zoomed out). It should get picked up when you reload the page (though default asset caching might hold it for another hour. Well, well)
Otherwise, this is a killer job :) Keep up this awesome project!