Install optipng by entering the following commands in the terminal:
sudo apt update sudo apt install optipng
Description:
advanced PNG (Portable Network Graphics) optimizer
OptiPNG is a PNG optimizer that recompresses the image files to a smaller size. It losslessly reduces the bit depth, the color type and the color palette of the image, runs a suite of compression methods and strategies, and selects the compression parameters that yield the smallest output file. It also recognizes several external file formats like BMP, GIF, TIFF and PNM (PBM, PGM, PPM).
Homepage: http://optipng.sourceforge.net/
Version: 0.7.6-1.1
Section: graphics