1. Download
Decal Converter
2. In your Cstrike folder delete tempdecal.wad and pldecal.wad AND custom.hpk and (if exists) custon2.hpk files.
3. Run the Decal Converter program, and open up the image you want.
4. Export it to your Cstrike directory.
5. Make a copy of this file in the same directory and rename it to tempdecal.wad.
6. Make both of these .wad files read only (right click, then properties, then check read only).
7. Create a LAN server to test it out.