The example image charlie.pgm is made available and modified under the terms of:
https://creativecommons.org/licenses/by-sa/2.0/
I could not find a clear name for credit, so I will link to my source on the web:
https://www.flickr.com/photos/tom-margie/1535413483
The image was converted to pgm format, but not other modifications were made.

The image rembrant-peale.ppm is listed as CC-0 or Public Domain

The image sleep.pgm was created by the author of this software based on an example
provided by the creators/maintainers of netpbm (it formerly said feep, I reworked it
to sleep - I was tired and needed to stop working). I consider this an independent
work and it is avialable in the public domain.