@theo h. mann They're image formats with very simple headers. They also have text modes in which you write the data with ascii decimal numbers which is convenient if you can't have your program write to a file because you can just copy and paste it into a text editor and save it.

https://gearlandia.haus/media/8ff03894b254fa9112581bdd172e01233e5ab49df854c66abdbc5aec8462724a.png