I'm not the author of this. I can't remember exactly where I got it, but I'm pretty sure it was inside either a firaxis fpk, or one from FFH.
This is a multi layered .psd file, designed for making button images creation easy. It can be opened in Adobe Photoshop, or Gimp. Possibly others that I don't know of too
When saving a button image as .dds, always use Explicit Alpha and no mipmaps