@need4spam
discussed before here: https://mangadex.org/thread/175032
Since that discussion I found: https://css-ig.net/pingo. Does a better job on PNG graphics. There is a GUI for running it. The author is a bit of an idiot. Some bugs (in GUI), and requires the sendto: right click to use and change the INI file that is used (read the comment section @site). Or you have to use drag and drop and reset values all the time.
For JPG types use nxpower lite desktop v8. It's a pay program but a google search can fix that. MAKE SURE TO DELETE THE PNG reduction program. nxpower lite is just another GUI program using shell out programs. It will then error on PNG types but it won't croak.
There you have it. And mangadex I don't foresee using reduction programs to save space. Any of the major scanlation groups already reduce prior to posting. I never see large reductions on mangadex, just sometimes.