"How do I apply a .jpeg image to a prop such as a cube surface in Poser6?" You use the material room. See screensots on this post. But remember if you want to add a picture to the whole cube, say for example turning it into a crate or cardboard box. You need to map the prop/figure using software like UV-mapper or lith unwrap.

Pinky - you left the lens cap of your mind on again.