See through textures?

How would I make a custom see-through texture, such as glass, or a decal which has spots that can be seen through? Any help is greatly appreciated. I have Paint.NET and VTFEdit, or Gimp.

Decals
[ul]
[li]Link 1 (Photoshop)
[/li][li]Link 2 (Gimp)
[/li][li]Link 3 (Photoshop)
[/li][/ul]
Transparency/Translucency
[ul]
[li]Link 1 (Gimp or Photoshop)
[/li][li]Link 2 (Photoshop)
[/li][li]Link 3 (Photoshop)
[/li][/ul]

Thanks ALOT.