I would recommend allowing people to draw without signing up. Or at least provide a demo that people can use before signing up.
Why not have the drawing pad on the homepage? That way you won't lose people, like me, who could not be bothered to sign up because it involved filling in a form.
Maybe have a save button and when they click on that force them to save. That way they have an incentive to sign up because otherwise they will lose their painting.
