Haven't heard of picross before, but I had a lot of fun with it. There's a bug after being World 3, Stage 1 where the screen goes black. Furthermore, some of the puzzles are impossible to solve "normally" (without looking several steps ahead, or trying something and backtracking if it doesn't work several steps later) and may have multiple solutions (only one of which is recognized as valid). I found one in stage 2, and two more in stage 4.
Only got to see 3/4ths of the story (since world 3 is unbeatable), but I really liked the art and writing as well. Would be interested in seeing the end to world 3.
Some other nitpicks:
- When completing a level, it transitions instantly, so I have no idea what image I completed.
- Having mouse centric controls would be nice: LMB for filling, RMB for X-ing, so it can be played with just the mouse.