I figured it out last evening after seeing what he did. I don't have it on my work computer, so I can't navigate you there directly... but it's under the 3rd or 4th option on the standard menu bar at the top. It's called Apply Overlay or something obvious like that. Zoom all the way out, apply it and then streatch it to fit. You might need to zoom in on the poles to get it snug. That seems to be about it.
I don't know what the ideal size/layout of the file you use should be. If your flat image should be distorted at the poles to appear not-distorted on the globe, etc.. Map projections are too advanced a topic for me right now. My image was sized to 12.5 inches width and 6.25 inches height, so that my earth-sized world would have have a scale of roughly 2000 miles per inch.
It worked, and I have to say I didn't fully realize how large some of my continents would look on a round world.