The identified pxlknot images I looked at were all 96dpi and 24bit color.

A general approach to decryption is to start with the simplest image, and then encode one character. Examine the resulting image. Do it again with the same characterto see if there is a change.

Then sequentially encode '1','2', '3', etc. and see if there is a predictable pattern.

What you're looking for is a way to brute-force decode the image.

Also try to find the original images before they were subjected to pxlknot.

Here is source code for determining entropy of a file. Can be used in connection with brute force decrypter to identify results with significantly different entropies.

https://pastebin.com/raw/Gx34MNZF