The *.png and README.original files in this directory are copied from libpng.org, specifically contrib/pngsuite/* in libpng-1.2.40.tar.gz. README.original gives the following license for those files: Permission to use, copy, and distribute these images for any purpose and without fee is hereby granted. The *.sng files in this directory were generated from the *.png files by the sng command-line tool.