TOVoptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections.
LinuxUserGD 
@textovervideo
Other tools:
https://trimage.org/
"A cross-platform tool for losslessly optimizing PNG and JPG files for web."
Other tools:
https://trimage.org/
"A cross-platform tool for losslessly optimizing PNG and JPG files for web."
@HugeGameArtGD Trimage looks like a GUI that uses optiPNG and a few other libraries. Trimage looks impressive.
lerudd@textovervideo just installed both (manjaro) and for the acid test, fed it a 252 MB png. a progress bar, even if standardly inaccurate, would be an improvement for trimage. it's still running after ten minutes and i'm going to assume this is too much for it :-) and terminate the process.