How does Steam set a game's background in the Library?

Lately, I realized, that the background of a game in the Steam Library changes from time to time, and I swear, I once even recognized the background somehow from a scene I've been to in a game.

So, how does Steam set the game's background?

Does it secretly take screen-shots while I am playing, or use preset images, or is it just random?

The background images for a game in your library (and also for the game's store page!*) are taken from the publisher / store page ^† and your gallery of recent screenshots.^‡

* If no background image was specified by whoever made the store page, it simply chooses the right-most screenshot from the game's gallery. Methodology: Looked at very old games on steam, then some new / popular games. Results held up to prediction on every case attempted.

† Images are taken from the store page only in the case that no images have specified by the publisher, that is.

‡ Methodology: I have some old pictures of The Guild 2 in my gallery, about half a year old. After clicking back and forth for new images in my library about 30 times, they've yet to show up as a background for the game.

To see if a newer picture would appear as a background, I loaded the game, and took a screenshot and uploaded to the cloud. It showed up the first attempt.

Similarly, taking another screenshot without uploading to the cloud also resulted in that image being posted as a background on first attempt.

Steam doesn't take screenshots by itsef. It uses those available for each game (including those you take yourself), when you select it in the library.

It uses preset images, but as soon as you made screenshots (via F12 usually), they will be used. At least that's what I experienced with Arkham Asylum