[a1369209993]

> So, after they're loaded and (possibly) scaled to fit the screen, I'd expect every background image to use exactly the same memory amount.

Well, not centered or tiled backgrounds, which I recall Windows did support, and upscaling could be done on a per-pixel basis (although I don't know if that was supported), but yes, the image format shouldn't matter.

[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.