OptiPNG is free tool PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections. OptiPNG is an easy to use command line tool for Windows.
There are 7 levels of optymalization files (2 is default). You can also use some advanced option offered by OptiPNG to strip metadata objects, run in simulation mode, run in quiet mode, log messages to file, cut one image out of multi-image or animation files, enforce writing of a new output file, keep a backup of the modified files and more. Example command: optipng -o5 filename.png