They're using a quantizer. Likely http://pngnq.sourceforge.net/ http://pngnq.sourceforge.net/ . To compare yourself: wget http://tinypng.org/images/example

They're using a quantizer. Likely http://pngnq.sourceforge.net/ http://pngnq.sourceforge.net/ . To compare yourself: wget http://tinypng.org/images/example-orig-c549c73a.png wget http://tinypng.org/images/example-shrunk-ec364794.png pngnq example-orig-c549c73a.png cmp -b example-orig-c549c73a-nq8.png example-shrunk-ec364794.png I get one byte of difference between what pngnq produces and their example shrunk version. BTW, I use pngnq all the time in instances where losing some information is okay. It's a great tool.