Everything is detailed in screenshots, just follow steps :D
- Picture 1 : Download a game background on gamebanana (manual download => download), save the file and go to Downloads directory in your file explorer
- Picture 2 : Right click on it, extract here (if you have no extraction option, get 7zip here or Winrar here)
- Open the folder that appeared in Downloads, if both .vtf files are in "whateverpackname?\materials\console" then you can copy the folder, if not, just create folders to get the same path.
- Paste it to your custom steam directory (C:\YOURNAME\Games?\steamapps\common\Counter-Strike Source\cstrike\custom) and you're ready to enjoy the background next time you launch your game!
If you can't find steam directory, open steam library, right click on CS:S, click on Properties,
go to local files tab, and click on browse local files !
I hope it helped you ! Enjoy your backgrounds :)