I've created an image slice/merge for any file supported by .NET. Why? It's much faster to go from mockup to borders than graphical import. Once you've set up the XML file you can run the utility from the command line. For example to create transparent borders, complete with alpha channel I run slicer to create the borders from my active/inactive mockups.
> slicer window-tx.xml
Where window-tx.xml is
This creates a top border 1024 x 29, a left border 9 x 730, a right border 9 x 730, and bottom border of 1024 x 9. If you add it all up, it's 1024 x 768 which is my mockup size.
Unfortunately, .NET 2.0 doesn't support Targa so I have a separate batch file to convert the PNG to TGA with the Open Source ImageMagick convert utility. Much faster than fumbling around with import. In a couple of seconds I can go from mockups to applying the style in WindowBlinds. Much easier to do buttons too as I can keep all glyphs in one image and let this utility slice it up.
My work is Apache 2.0 Licensed so if you're interested, leave a comment.