The core idea for the app is fun and playful - a totally different way of making compositions.
The UX could use some improvement - helping me understand how to use it, what the buttons do (eg two different sized squares without labels for grid size isn't very helpful). Camera front or back should be a camera icon button instead of giant text label that doesn't look like a button.
Once I start painting or erasing, I want to remain in that mode, even if my finger goes outside of the image area. Currently I find myself accidentally switching to erase mode and messing up my progress.