If you want a good list of command-line tools to do this sort of thing, have a look at the ImageOptim website (http://imageoptim.com/), which lists all of the command-line tools it combines together.
I have an image that imageoptim brought down to 149kb. I ran THAT already compressed image through compressor.io and it brought it down to 51kb. Whoa. Very very minimal degradation in some of the image drop shadows but not enough to care based on that file size