How does GTmetrix optimize my sites images?

[manic]

It's not smush.it, that doesn't optimize as well.
Looking to see what gtmetrix uses, can't seem to find it on their website.

[Jake]

Just glancing, it seems to do an analysis and recommend compression methods, but doesn't not do the smushing on it's own. The right hand side of this page discusses compressors
http://gtmetrix.com/optimize-images.html

BTW I believe some jpeg compressors do a smarter job for others, not creating a preview icon is another way to save a tiny bit, Photoshop export for web function does an extra hood job.