That's the dilemma that I ran into, plus I kind of felt that I was categorizing all of the creatures as different colors in Magic: the Gathering.
So, my working theory has been, leave the background black, then the designer could paste the subject onto whatever background they wanted.
(Of course, I realize that most designers will just use the picture as is, even if I were to include a manual with each one on how to customize it for their own use. Why? Because, that's what happened in FRUA.
)