Just as long as they remember it's satire, and the Imperium are not good guys (and nor is anyone else). Maybe they can remind Games Workshop of that too
Search for creative commons or public domain artwork - Wikimedia Commons is a good start, or a google image search (under the image search bar select tools -> usage rights -> creative commons rights)
Or don't worry about the copyright issues of AI and learn to use that to create things you want - I suggest SDXL is the best option
It might have been nice if he had worked out literally anything on his own, but as everything else about the film except Colin Firth's Penguin (who was only in it briefly) was dire I don;t think could have saved it