[kimixa]

I don't know the implementation details - but I wouldn't be surprised if it kept a full-screen-size already-scaled/tiled/centered image somewhere in memory

Back when win95 and co were being made, decoding an image was a lot of work for the hardware, I suspect they'll try to make restoring the background (e.g. dragging a window around and changing what's occluded) to be the simplest bitblt they can.