Sorry, I wasn’t actually asking so much as trying to make the point that since the process of how cards work isn’t detailed anywhere, there’s a ton we don’t know that also applies to the combo cards, so we might not have an answer to @geewhiz’s question.
Also, I just checked and there are a bunch of backgrounds in the photo editor. I think you should be pretty pleased how with how it compares to XCOM 2’s posters.