And some shots of the wireframes and lowpoly with no diffuse applied so you can see the sculpt more clearly... Lowpoly - No Diffuse Lowpoly - Wireframes
Diffuse has strange color. Spec has no variation. Also your lightning is pretty basic. Try to change that yellow on diffuse, to something less saturated. Try to bright the light up, and maybe increase the radius. Try to change diffuse boost to higher value.
I must have misunderstood the question. I took it as will this "single image as a diffuse work as a diffuse and spec map in my 3D package"? That's why I mentioned shaders. The engine the art test is for must have a shader that uses the diffuse alpha as a specular map.
A developper can create a diffuse with many others diffuses map. With this method, you can create multiples diffuses maps like layers for photoshop. So in-game script can add them with parameters (manipulating colors, scale, rotation) the limit is only the imagination.
What is the correct name for the maps that accompany the diffuse map? E.G. spec, normal, emissive. I've been referring to these for years as support maps, I just made that up though. I heard a CG Supervisor refer to these once as attribute maps. Is there a correct technical term for all the other maps you need to support…