While the game has around 600 standard puzzles and the 2 Murals, there's another mode available which will allow for you to do some small Picross puzzles; Professor Tetra's Daily Training. These puzzles are set by Professor Tetra and are relatively simple but increase in difficulty and have to be done in consecutive bursts of multiple puzzles.
When the puzzle is completed, Professor Tetra will give you Picrites and the Puzzle Level will increase. The amount of Picrites you earn is based on the progress you have taken in the game and the level of the daily training. This is a good way to get Picrites.