I'm working on some frames that pretty closely match your description of what you are trying to build. My pictures are prints on metal substrate, but other than that the floating frame look is what I'm going for, and they will be black. I'm using poplar because it is more stable than some other hardwoods and I have a bunch left over from some other projects. I like the way the poplar machines and how well it takes paint plus it is easier on my cutting edges than other hardwoods.