Well, here is my two cents. I was able to do this for both CS-Source and Condition Zero by making a bmp file in photoshop and then importing it into CZ using Wally.
When you're in your favorite photo imaging program, set the size of the image to 128x128 and then paint your heart out to make it personal. Save the file as a bmp file. Then go to
http://www.telefragged.com/wally/ and download it. Now open Wally and go to the menu's and pick Wizard, then "hl color decal". In your "Please select your half-life folder" enter the following path: C:\program files\steam\steamapps\youremail@yourisp.net\condition zero\czero
And in the "File on hard drive" box, enter the path to the bmp file you built and click OK. Now go into Windows Explorer to that path above and you will find that Wally built you a file called "pldecal.wad" which is not named correctly for Condition Zero. Look for a file called "tempdecal.wad" in the same folder. If it's there then rename it to tempdecal.old and then rename pldecal.wad to tempdecal.wad. You're done. Now when you load CZ and go to options you will still see your old spray selected, but ignore it. When you spray in CZ your custom spray will spray.
In CS-Source this process is super simple. Take the bmp file you created and convert it to jpg with your photo program. Now load CS-Source and drive it to your jpg file and select it. Done.
