The draw9patch.bat tool is great.
It makes a very little-size image can strech all over the component (or the screen).
It is placed in the SDK tools dir.
We can only point a point at the image's top,left,right,bottom and it will be very usefull.
Also,we don't have to save the image as a *.9.png file. Just save it as a *.png file will be helpfull too.