iphone - iOS 5 custom tab bar image vertical alignment

Question

I'm getting some odd behaviour with my custom tab bar. The images seem to be aligned incorrectly. Here is a screenshot (I have removed my own tab bar background to highlight my problem):

Here is the code I'm using to set the images for each state:

self.tabBarController = [[[UITabBarController alloc] init] autorelease];
self.tabBarController.viewControllers = [NSArray arrayWithObjects:homeNavController, whatsOnNavController, mapNavController, infoNavController, nil];
self.tabBarController.delegate = self;

// For iOS 5 only - custom tabs
if ([self.tabBarController.tabBar respondsToSelector:@selector(selectedImageTintColor)]) 
{

    // Set the background images
    //[[UITabBar appearance] setBackgroundImage: [UIImage imageNamed:@"nav_bg.png"]];
    [[UITabBar appearance] setSelectionIndicatorImage:[UIImage imageNamed:@"nav_over.png"]];

    [homeNavController.tabBarItem setFinishedSelectedImage:[UIImage imageNamed:@"nav_home_over"] withFinishedUnselectedImage:[UIImage imageNamed:@"nav_home"]];
    [whatsOnNavController.tabBarItem setFinishedSelectedImage:[UIImage imageNamed:@"nav_whats_on_over"] withFinishedUnselectedImage:[UIImage imageNamed:@"nav_whats_on"]];
    [mapNavController.tabBarItem setFinishedSelectedImage:[UIImage imageNamed:@"nav_map_over"] withFinishedUnselectedImage:[UIImage imageNamed:@"nav_map"]];
    [infoNavController.tabBarItem setFinishedSelectedImage:[UIImage imageNamed:@"nav_info_over"] withFinishedUnselectedImage:[UIImage imageNamed:@"nav_info"]];

}

All of my replacement tab images are correctly sized (49 pixels high and 80 pixels wide for the non-retina versions).

What could be causing this odd behaviour?

--- Update ---

Here is an updated screenshot with the background in place:

Answer 1

There is a property on UIBarItem (UIBarButton item inherits from this class) imageInsets.

To use full height images (49px) for finishedSelectedImage and finishedUnselectedImage you need to set these image insets:

tabBarItem.imageInsets = UIEdgeInsetsMake(6, 0, -6, 0);