Its a good question. I built a little script using javascript, that automatically generated an image for each card using dall-e's api and dumped the image into a folder for me. Then I built a little work flow with Affinity Publisher, where I can use a template and a csv file of monsters and it will build the deck from the csv and the folder of images.
Ultimately, I did it with AI just as a matter of convenience. I also have a card game idea that I wanted to build using this workflow. This deck was pretty much a test bed to build and test the work flow.
But public domain would be find.