I scanned a few comics, and have images like this
I would like to have the background white ... in fact, I would like to have the white transparent, but that's just "nice to have", not "must have".
The problem is that there is some use of greyscale, such as
at least, I think that his "beard" is ggreyscale rather than just black
Which gratis software can make this presentable? If it helps, I am thinking of printing T-shirts.
Preferably Windows, but any o/s is acceptable