给navigationbar设置背景图片，并且按比例延伸，显示完整图片。

//按比例显示图片
UIImage *backGroundImage = [UIImage imageNamed:@”titleImage”];
backGroundImage = [backGroundImage resizableImageWithCapInsets:UIEdgeInsetsZero resizingMode:UIImageResizingModeStretch];
[self.navigationController.navigationBar setBackgroundImage:backGroundImage forBarMetrics:UIBarMetricsDefault];