Set image type to Sliced for buttons in GUI skin?

I can't figure out how to assign sliced images to my GUI buttons.

I'm using a GUIskin which is showing my Texture on my buttons. But it's simply stretching the texture and not slicing it.

The texture slices fine when I create a button and drag the texture into the button's Image Script 'Source image' and set 'Image Type' to sliced. So there's nothing wrong with the texture.

I can't find a way to tell my GUIskin that the texture is supposed to slice.

I'd really like to use GUIskin to keep things simple and be able to experiment with visuals easily. Is it possible?