ffmpeg batch command to reduce file quality

I've got a directory with thousands of images (png, jpg, bmp, etc.) and thousands of videos (mp4, mpv, mpeg, etc.). The png images may be ~10 MB and I can open them one at a time in GIMP, reduce quality from 100% down to 92% and the image size goes down to ~2MB and the quality (to the eye) hardly changes. How can I do this process in bulk (ie batch) to everything (pics and video) in the directory using ffmpeg? And how do I overwrite everything in the directory so I don't have duplicates? Are there any GUI tools for such a thing? Maybe it's better to stick with the command line.