initializing a UIBarButtonItem with a UIImage and resizing the image in iOS

I am trying to initialize a UIBar开发者_运维技巧ButtonItem with an Image. The available method is initWithImage. When I do this, the image is just a block of color. I want to be able to resize this image. How do I do that?

Internal UIBarButtonItem images need to be one solid color (system turns it into white if you don't) with an alpha channel for transparency.