I use ImageOptim. It’s free, open source, quick and easy. I drag and drop the images into it, and it just compresses them. Boom. Done. Be careful though! It just overwrites the original files with the optimized versions by default. So make copies of your images BEFORE you drop the files in, otherwise you could lose your original image quality forever.

Then I can upload those images to my WordPress website without taking up too much disk space, and most importantly, speeding up page loads.