Standard .PNG image with an extra 1-pixel-wide border, which allows the user to define its stretchable and drawable areas when edited; Android automatically resizes the file based on screen size to best present its contents.
NinePatchDrawable images are often created/edited with the Draw 9-patch tool found in Google Android SDK. This file is frequently used by Android Developers to help create the user interface. The Draw 9-patch tool enables the user to create an image defined by 9 areas or patches: 4 corners, 4 edges, and the middle area. The user can edit the 9 patches to reflect how they want the image to be stretched. Also, the file must be saved into the res/drawable/ directory of the project.