Basically, I just tried to find out myself how to create pretty good quality buy menu images, and I DID find a way to do it and end up with a good result in-game.
I'M NOT saying this is the best, quickest, easiest way to do it, but it works, so I thought I would share it with you.
- Gimp download
- VTFedit download
Feel free to ask questions!
ps. Use 'counter-strike source/bin/hlmv.bat' instead of the SDK; to install it, put it in 'steamapps/common/counter-strike source/cstrike/custom/my\_custom\_skins/materials' instead of just 'cstrike/materials'