The author says:
Softcover non-fiction book providing a photo study of road signs in use by 21st Army Group in Northwest Europe during the Second World War.
Well, that’s definitely niche.
I think I would up the historical/vintage character of the photos — instead of pristine back-and-white photos overlapping at perfect right angles, I’d have photos on yellowed paper with quarter-inch borders showing scratches and foxing, placed as if they were physically arranged on a background of khaki canvas or worn leather. Similarly, the stenciled letters of the title wouldn’t be stark and perfect black-and-white, but a greenish off-white on a dark painted wood surface (or the same khaki or leather extending from beneath). You would still be showing off old images, but you’d do so in a way that also gives your cover some warmth and character.
(And chop the number of images on your cover in half, at least.)