Find game which a Steam profile background is from?

How do I see which game a Steam background is from?

For example this background image: http://media.steampowered.com/steamcommunity/public/images/items/620/904c34bd20f5a5f5eadb074cce42d2606b266acc.jpg

The easier way is to look at the game ID:

http://media.steampowered.com/steamcommunity/public/images/items/620/904c34bd20f5a5f5eadb074cce42d2606b266acc.jpg

If you put that number into this url, you go to the game page:

http://store.steampowered.com/app/620/

Game 620 on steam is Portal 2.

Keavon also suggested searching for the appid on http://steamdb.info/, which seems to work quite well.

Adding onto GnomeSlice's answer for a sure-fire way to find it:

Right click the user's profile in a browser and click View Background Image. Copy this image's URL and do a Google Images search by image for the background's URL...

Then remove any text from the search, but not the image, and add site:steamcardexchange.net to the search...

This should show you the result on Steam Card Exchange's website for the game with that background...

That page is the showcase for all things related to Trading Cards and the game with the background. This should work for pretty much any background, no matter how obscure it is.

If you're using chrome, you can right-click the image and select 'Search Google for this image".

That particular background is from Portal 2.