If you wish to see the code I use, click here

The Atom logo shows how it is possible to scale the image to be smaller or larger by changing the size of it through background-size so it really is quite useful.

The F-Zero GX cover has a background-color set to black since it's the general theme of all the websites it has. IncludingCrazy Game Nerd's website.
(Unlike mine, it is very much not mobile friendly)

It also uses a different setup to most of them, using a lower resolution image and then switching to a higher resolution after you enable it. It, of course, requires user interaction - not only do you have to click the image, but you also have to be hovering over it to enable the high resolution, saving some bandwidth. No much, but enough to be worth the extra effort.