When we took over Horse & Musket: Dawn of an Era, it had a sort of cover already, which designer Sean Chick used to advertise it on BGG.
It's a very nice painting, and one that evokes the period - in fact, we used it on the cover of the rulebook - but wasn't really our style; we're not "nice painting on the cover" sort of publishers. We don't mind incorporating period artwork or photos into our box cover designs, and we do that more often than not; we just don't want to let that artwork do all the heavy lifting.
My first instinct was to get some illustrations from a period military manual, as that'd be something unmistakably from the period, and also perhaps a little grittier and less "handsome uniforms, cool hats, ridiculous wigs". I thought I had found what I was looking for, and sent a first draft cover to Sean:
Sean pointed out, however, that this wasn't actually from the period covered by the game! The illustration was from the previous century! (Darn you, internet; you lied to me.) But Sean saw the kind of thing we were going for, and suggested an alternative - a copperplate engraving of Eugene crossing the Alps. Yes, that would work - that would work like gangbusters.
The cover purposefully mimicked a sort of book cover. I used a portrait rather than our usual landscape orientation. We also had the author's name centered and in large text. I think this made Sean a little self-conscious, because he requested it be made a little smaller. In retrospect, it had thrown off the balance of the design; the whole thing is a lot more harmonious in the final product: