Seven years ago in 2012 I spoke at RubyConf China 2012. It was a technical talk on how to become a better developer, if you're interested…
From my experience, PunyPNG produces better results than Smush.it does.
The sprite image for this blog yields 160kb when it was created. Smush.it reduced the size by 30%, PunyPNG reduced the size by 50%.
The PunyPNG API is going to be extremely useful when it is out of beta.