http://dplogin.com/forums/index.php?topic=2428.0 (Link is NOT broken, search next time)
The guide might not be detailed enough for you, so here's a quick step-by-step guide.
Things You'll Need:
- a picture to convert
- a program to edit the picture
- wally (
http://www.filefront.com/3632507/This-is-wally-version-1.55b/)
1. Save a picture you want to convert into a texture.
2. Use any picture-editting program to open the picture.
3. Edit the picture size (note: not canvas size) so that its' dimensions are evenly divisble by 16. (In this case, primary dimensions include 64x64, 128x128, 256x256 and 512x512. Whch dimension you use affects the "resolution" of your texture. The smaller the size, the lower the resolution.)
4. Save your resized picture into a .JPG file.
5. Open Wally and open your picture file.
6. Go to File, Export, and select Export Type as Quake 2 Texture (.wal)
7. Put your new .wal file into a folder with a specific name (such as mytextures) and put it into C:\bsp\Paintball2\quake2\pball\textures
8. Put your resized picture file into a folder with the same folder name, and put it into C:\Games\Paintball2\pball\textures
9. Load up BSP, and you'll be able to load up your new textures.