UINavigationBarの背景画像を設定する方法

#import <QuartzCore/QuartzCore.h>

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    UIViewController *vc = [[UIViewController alloc] initWithNibName:@"ExampleView" bundle:nil];
    UINavigationController *nc = [[UINavigationController alloc] initWithRootViewController:vc];
    nc.navigationBar.tintColor = [UIColor blackColor];//画像の色に合う色にした方が良いと思う

    NSString *currentVersion = [[UIDevice currentDevice] systemVersion];
    if ([currentSystemVersion compare:@"5.0" options:NSNumericSearch] == NSOrderedAscending) {
        // iOS5.0以前の場合
        UIImageView *navBg = [[UIImageView alloc] initWithImage:[[UIImage imageNamed:@"bg-nav.png"] stretchableImageWithLeftCapWidth:0 topCapHeight:1]];
        navBg.layer.zPosition = -FLT_MAX;
        [nc.navigationBar insertSubView:navBg atIndex:0];
    } else if (([currentSystemVersion compare:@"5.0" options:NSNumericSearch] == NSOrderedSame) || ([currentSystemVersion compare:@"5.0" options:NSNumericSearch] == NSOrderedDescending)) {
        // iOS5.0以降の場合
        nc.navigationBar setBackgroundImage:[UIImage imageNamed:@"bg-nav.png"] forBarMetrics:UIBarMetricsDefault];
    }

}