From google: If your app uses a custom image as the background of the bar, you'll need to provide a “taller” image so that it extends up behind the status bar. The height of navigation bar is changed from 44 points (88 pixels) to 64 points (128 pixels).
一句话：将原来的44(88)像素的背景图片改为64(128)像素高度的图片即可。