PNG is a raster-graphics file format that supports lossless data compression. - wikipedia

PNG supports palette-based images, and full-color non-palette-based RGB or RGBA images.

Optimizing tools

Before zopflipng was available, a good way in practice to perform a png optimization is to use a combination of 2 tools in sequence for optimal compression: one which optimizes filters (and removes ancillary chunks), and one which optimizes DEFLATE. Although pngout offers both, only one type of filter can be specified in a single run, therefore it can be used with a wrapper tool or in combination with optipng or pngcrush, acting as a re-deflater, like advdef.

• OptiPNG - overwrite file in place

optipng file1 [file2 ...]

• pngcrush

pngcrush -brute -rem gAMA -rem cHRM -rem iCCP -rem sRGB -rem alla -rem text  input.png out.png

• Pngquant

see also

• The smallest 256x256 single-color PNG file, and where you’ve seen it (2015)
• The world’s smallest PNG / HN - is 67 bytes. It’s a single black pixel.
  • What’s the smallest file size for a 1 pixel image? / HN - echo -n ‘P5 1 1 1 ‘ > a.pgm - 10 bytes pgm