Setting a background image for a tabbar

I'm trying to programmatically set the background image for a tabbar in my app. My code is as follows:

RootViewController.h

IBOutlet UITabBar *mainTabBar;
    IBOutlet UITabBarItem *settingsBarItem;
    IBOutlet UITabBarItem *infoBarItem;
    IBOutlet UITabBarItem *aboutBarItem;

RootViewController.m

-(void)viewDidLoad {

    UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"smallMenuBackground.png"]];    
    [mainTabBar insertSubview:imageView atIndex:0];
    [imageView release];

    [super viewDidLoad];
}

This is not working for me.

UPDATE

UPDATE 23rd January 2012

Ok, I've made a bit of progress. This only stopped working since I upgraded to Xcode 4.2 and IOS5. I managed to get it back using the options in Interface Builder, but now it only works for IOS5. Ideally I would have liked to get working programatically but I'll settle for the IB solution for now.

I just can't seem to get it working for any previous releases.

NOTE: my TabBar is only on my RootViewController, which is the main screen of my app.

Ideally, if I could get the code working that Nithin suggested, that would be great:

UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"tabBG.png"]];

if ([[[UIDevice currentDevice] systemVersion] floatValue] > 4.9) {
    //iOS 5
    [self.tabBarController.tabBar insertSubview:imageView atIndex:1];
}
else {
    //iOS 4.whatever and below
    [self.tabBarController.tabBar insertSubview:imageView atIndex:0];
}

[imageView release];

Any help would be appreciated.

Regards, Stephen