Things You'll Need
Instructions
Select the image and copy it to the C:\Program Files\Steam\SteamApps\(Your Email)\(Counter-Strike Source) directory.
Launch the game and click on the "Options" tab. Then select the "Multiplayer" tab and click "Import Spray".
Select the image you copied earlier and click on "Open." You should see a preview in the options window.
Click "OK" and launch the game.
Walk up to a wall in the game and hit your spray paint key. You will see your custom image painted on the wall.