Honestly, I don't consider PNG a simple format. The CRC and the compression are non-trivial. If you're using a new language that doesn't have those features built in and/or you don't have a reasonable amount of programming experience then you're going to likely fail (or learn a ton). zlib is 23k lines. "simple" is not word I'd use to describe PNG
Simple formats are like certain forms of .TGA and .BMP. A simple header and then the pixel data. No CRCs, no compression. Done. You can write an entire reader in 20-30 lines of code and a writer in other 20-30 lines of code as well. Both of those formats have options that can probably make them more work but if you're storing 24bit "True color" or 32 bit "true color + alpha" then they are way easier formats.
Of course they're not common formats so you're stuck with complex formats like PNG
Simple formats are like certain forms of .TGA and .BMP. A simple header and then the pixel data. No CRCs, no compression. Done. You can write an entire reader in 20-30 lines of code and a writer in other 20-30 lines of code as well. Both of those formats have options that can probably make them more work but if you're storing 24bit "True color" or 32 bit "true color + alpha" then they are way easier formats.
Of course they're not common formats so you're stuck with complex formats like PNG