Hey all 
Last week I’ve been working on my company site - https://voucherify.io
to make it a little bit faster. It turned out, that most of our thumbnail images were full fat .PNGs, which aren’t efficent enough. We decided to change them for JPGs.
Programatically
We’ve downloaded each image, resized it, and then put it back online.
See the repo below for the app.
Feel free to fork the code or point out any bug