1. a. 512x512 pixels(not an exact requirement, but a mere suggestion for best image quality)
b. .JPEG, .tga, .bmp, .gif or .vtf
2. Place the picture file in your cstrike folder located at: C:\Program Files (x86)\Steam\SteamApps\common\counter-strike source\cstrike\materials\vgui\logos or C:\Program Files\Steam\SteamApps\common\counter-strike source\custom\[custom_name]\materials\vgui\logos
Note: If it's a .tga/.jpg/.bmp/.gif save it in your CSTRIKE folder, also put a copy of any .vtf there.
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.
Full tutorial here: http://css.gamebanana.com/tuts/215