It looks like there's a lot missing. There's a whole lot of negative space in there. Think of the art found on US currency - and I'm not talking about the fancy seals and curlicues. For example, on the back of the current $2, there's a picture of the signing of the Declaration of Independence. I think you'd be better served with some kind of scene in a similar style and using black, blue and red for the backs. My suggestion of a battle scene with two battalions facing off, charging the battlefield, would fit here very nicely and give you a lot of cool-looking detail reminiscent of the older, classic card backs.