How to center images vertically in the UITabBar

I have a UITabBar without titles and I need to center tabs images vertically. How I can to this?开发者_如何学C I know that I have to subclass the UITabBar, but I have no idea how.

If the only reason you want to subclass UITabBar is to have an image only tab bar you might be better off with one of the custom tab bar controllers from cocoacontrols