The only things I would recommend are:
- The help screen in the beginning, doesn't look like a modal. It took me ages to actually figure out there was an "x" button in the top-right. Maybe add a darken behind the help popup, to show that it can be exited.
- "Show Grid-lines" should be a first class option. I couldn't easily find out how to show grid-lines and without you're clicking in the dark to find where the pattern begins and ends.
These are just some small UX improvements, overall it's an impressive tool and I wish you all the best!
- make the prompt appear inside the trash button and not outside it. That should be enough to make it more obvious. If not enough, then add a small text to it.
like this maybe:
What you see is not what you get.
Try to see the tiled image or exported to zazzle, and lines are no longer continuous :(
(By the way I also just notice sharing a zazzle url like : https://www.zazzle.com/eschersketch_wrapping_paper-256519193... is problematic because it is very tempting for the user to do so, but everyone is sharing the same link, and what is displayed have some caching issues), (Zazzle also gives a warning when the image is too big, Zazzle is also really expensive 33€ for 76cm x 1.80m)
If you can sort the above issues, you can probably easily add some fractals symmetries :)
I once made a similar thing, but it was more of a creative-play-thing for children (so no way to create tiled graphics, but hopefully easy to use and fun to play with). It is at: http://kaleidopaint.com/