Apple has added a tiny line over the tabBar in iOS 7 which is supposed to work as a shadow or fade between the tabBar and the UI
I'm not seeing anything in the UITabBar
API for affecting that separator, but if the separator is within the UITabBar (a UIView subclass), I'd expect you can insert a new one-pixel-high UIView on top of it. You'd have to grab a slice of the image that you want to appear there and draw it in the new view. And I'm not sure if UITabBar would somehow prevent adding the subview or prevent the subview from being on top. But that's where I'd start.