If you use iOS4, you can override drawRect
@implementation UINavigationBar (UINavigationBarCategory)
-(void)drawRect:(CGRect)rect
{
    UIImageView *itleImageView = //create object
    [self addSubView:titleImageView];
     //set title view in navigation bar
}
@end
iOS 5,
if ([[UIDevice currentDevice].systemVersion floatValue] >= 5.0) {
    if ([self.navigationController.navigationBar respondsToSelector:@selector( setBackgroundImage:forBarMetrics:)]){
   UIImageView *itleImageView = //create object
   [self.navigationController.navigationBar addSubView:titleImageView];
    }
} 
i guess this is a right, as i don't have implemented.