The 9 Patch drawable technique is rather logical, maybe it isn't explained well in the official Android docs, basically you have to define a central area of your image that can be scaled. Check my attachements, a source image I created in 30 sec and a 9 patch drawable I've created using this site:
http://android-ui-utils.googlecode.com/hg/asset-studio/dist/nine-patches.html Get a designer! They can create the source image and you can use the tool to generate your own 9 patch drawables. You can also do that in your favorite image editor and draw the black pixels yourself if that website helped you understand the concept better.