XnView Software

Image > Adjust... > Levels... And also possibly: Effect > Effect... > Map -- Saturation
The Levels option is assigned the useful keyboard shortcut 'L'.
I normally convert nominally black-and-white images to grayscale before using Levels, let me know if you know a better method!
Due to the ...

Until now, I was using Irfanview in Windows and with Wine. I no longer want to use a Windows software in Linux, and want to change to using XnView.

I clean up and restore public domain drawings and prints, mostly black & white. The most important tool is the color replacer in 0 to 255 grayscale ...