Installing a Custom Spray for Source Games

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=390

To install a custom spray for Half-Life 2: Deathmatch or Counter-Strike: Source, you first need a valid image file. This can be either a .jpg, .bmp, .tga, or .vtf file.

Once you have your image saved somewhere, go to Options > Multiplayer. Then look under "Spraypaint image", and push the "Import Spray..." button. Browse to where your image is located and hit open.

You should now see your custom spray in the window, which means it's ready to be used.

Setting a spray only takes affect when you join a server, so if you change a spray while in game, you will have to disconnect and rejoin for your new spray to be used.

Sprays are not available in single-player games.