This was a lot of fun. I completely all the puzzles on normal difficulty. I will admit that I did take advantage of the pieces lighting up when placed in the right spot a few times when I got stuck, particularly on some of the puzzles where the colors were very close to each other. I liked that you had the option to show the borders and incorrect tile rims, but that they were off by default. The other menu options were nice to have as well.
On the puzzle "mandelbrot adjacent", when the menu was open I couldn't see all of the puzzle. I hadn't realized this at first, so was very confused as to why the picture wasn't matching up right. You may want to scale the image down when the menu is open, particular on some of the wider puzzles.
I was amused that the "You Win" was not the final puzzle. When I solved it, I got the "You did it. Every. Puzzle. Complete." text, even though I still had more puzzles left.
I played this in two sittings. I did all the puzzles that were there yesterday, opened it up today to write my review to find new puzzles (yay!), and went through the new ones. As such, it's a little harder to be certain on the difficulty progression, but overall I think it was alright. There were a few of the later puzzles that I found easier than a few of the earlier one, though I think it was as much the fact that certain pictures simply made more sense in my head than others. I like that completing a puzzle unlocked more than just the next one, because it allows for folks to skip one that just isn't clicking for them.
I really enjoyed playing this and will likely come back to see if you add any more images in the future.