It's pretty obvious how it's done.
Create said art in a program that supports tga. You could also download BLP2PNG and then create the files and save them as png. Then use that program to convert which turns them into blp.
The 2 main graphic programs that people use that support tga..
- Photoshop (not free unless you know where to dl)
- Gimp (free)
Most if not all graphic programs support png.
Here's some helpful information with textures:
https://forums.wowace.com/showthread.php?t=15439