Hello IgorG, and thank you kindly for your reply.
I'm currently using a WordPress plugin to do just that, but it's doing a lousy job at it, because they all want me to pay an annual bill for processing all the images on their servers. I am looking for an open source tool that I can run on my own server that automatically compresses image files or my self-hosted CDN.
I'm thinking about using a tool like jpegoptim and cronjobs,
0 2 * * 0 lockrun -Q -L .lockjpegoptim -- find <path> -iname *.jpg -exec jpegoptim --strip-all -p {} \;
0 3 * * 0 lockrun -Q -L .lockoptipng -- find <path> -iname *.png -exec optipng -o7 -preserve {} \;
This cron should only affect .JPG en .PNG images modified in the last 7 days that are stored in my CDN, so the original should be preserved.