ImageOptim is not just for compression, it is for stripping away useless metadata, color palettes and other tricks.

For optimal images, you could do something like this:

1) Resize the image (And apply the trick from this article) use the proper quality marker (anywhere between 85-95)

2) Run it through Imageoptim

If you want a commandline version of ImageOptim, I have had good results with https://github.com/toy/image_optim