The app looks nice but it has usability issues.
I downloaded the app and used it for a bit but wasn't able to figure out how to do anything besides clear the page by swiping the grabber up and so I deleted it. I hadn't read the website nor the app description and there was nothing in the app itself to indicate that I should double-tap to bring up the action sheet. (Note that in iOS in general double-tapping is typically used to zoom.)
Here are two possible solutions.
(a) on first launch show a modal dialog that says something like "double-tap the grabber to access the menu"
or maybe a better fix:
(b) have the options "behind" the paper so when the user slides the grabber up the options become visible, then if the user continues to swipe all the way up the screen, the drawing is cleared.
I'd love to chat with you about the app. Post a link to your twitter/email/smoke signal/etc if you're interested.