1. Find an image that is of the correct size.
2. Find a program that will convert it to .wad
3. Place the .wad file in your cstrike directory
4. Rename the .wad file to tempdecal.wad or decal.wad
5. DO NOT GO INTO YOUR SPRAY PREFERENCES IN GAME OR IT WILL RESET IT
6. Spray the spray
7. ???
8. PROFIT
This assumes you play the lameness that is CS 1.6. In the greatness that is Sauce, all you have to do is go to options, click import spray, and then find the image with the browser. Sauce pwns.