For me, this is the quickest way to convert and optimize a bunch of images. This example converts a bunch of PNGs in JPGs and resizes them to a width of 1100 pixels. It then opens ImageOptim to further optimize the output.

1. Install ImageOptim
2. Install ImageMagick – if you have Brew the quickest way is: brew install imagemagick
3. Run this command: mogrify -format jpg *.png && mogrify -resize 1100 *.jpg && open -a ImageOptim .

Be aware that mogrify replaces the original file – use convert if you want to keep the original files.