I am also trying to figure this out... I have tried decompiling with a lz4 decompiler from github. no luck... i have tried to change the file extension to lz4 from pkm with no luck either... I tried numerous pkm file viewer and none are able to view the file.. there has to be someway as android (the game) is able to do it.. could there be some sort of answer in the .apk how it is able to decompile and view the image?
If anyone can help I have included a sample file so you can try yourself / view more information about the file.
https://drive.google.com/file/d/1x9yHHRunhksRbaTy8cMwIua8f0R9tdlL/view?usp=sharing
Thank you