1.Download/Save picture: Whether it is a picture you liked over the Internet, one you made, you can use any image as a spray as long as it follows this simple rules:
a. 256x256 pixels(not an exact requirement, but a mere suggestion for best image quality)
b. .JPEG, .tga,.bmp or .vtf
2. Place the picture file in your cstrike folder located at:
C:\Program Files\Valve\Steam\SteamApps\yourname\counter-strike source\cstrike
3. Start CS:S and click on options.
4.Click on Import Spray and then on the cstrike folder.
You will see the name of the picture file; select it, click open and then click OK
5.Connect to your favorite server
6.Press T (default key) for spray and notice the picture.
7. Have fun!
P.S: If you change the spray while playing, you will have to reconnect to the server you're in, in order to see the change. I hope this helps.