I made a compress image with OptiPNG v0.6.4
I used this command line: optipng -o3 -i0 -log=optipng.log img11.png
If first execution, OptiPNG say \"8 bits/pixel, 256 colors in palette. Reducing image to 8 bits/pixel, grayscale\"
In second execution, OptiPNG say \"8 bits/pixel, grayscale. Reducing image to 2 bits/pixel, 4 colors in palette\"
The same happend with -o2 to -o7
I attach the log file, and the image without compress.