Excerpt from new Android Training by Google:
Supporting different screen sizes usually means that your image resources must also be capable of adapting to different sizes. For example, a button background must fit whichever button shape it is applied to.
If you use simple images on components that can change size, you will quickly notice that the results are somewhat less than impressive, since the runtime will stretch or shrink your images uniformly. The solution is using nine-patch bitmaps, which are specially formatted PNG files that indicate which areas can and cannot be stretched.
Therefore, when designing bitmaps that will be used on components with variable size, always use nine-patches.
