Allyson Longuiera is the publisher at WMG Publishing. She also has a Masters in design, and does their covers.
In her publisher blog this week, she talks about using stock art in cover branding, and shows how she took two appropriate but disparate-looking pieces of art and made them match each other well enough to help cover-brand the first two books in a series.
It's one of those things that's simple when you read about it, but I probably wouldn't have thought of it. Check it out.