iPhone Tab Bar Control Icon Problem

I have a problem using icons with the tab bar control and I don't know to fix. The icons I'm using are png format 24*24.

They have a grey gradi开发者_如何学Goent background with a black shape in the middle. The problem I have is that only the background gradient shows. The Black shape in the middle does not.

Have I Used the wrong format or something? I read in a book to use png format size 24*24

You can't use a background on your icon.

The icons are designed to be used like an alpha mask and background will make it look like a filled square. Remove the gradient and use an image with an icon in the middle and a transparent background.