Draw Button with GUITexture which looks like Unity button

Hey Guys, I was wondering how unity draws buttons (if drawn in the OnGUI function). When I look into a GUI skin, the default button background is an image with only 12x12 pixels. I now want to use the same image with a GUITexture. But because the size is 12x12 pixels, the texture becomes ugly and pixelated if I increase the size of the GUITexture. So how Unity is able to draw a good looking button from this small image, no matter how big you want the button? And how can I do the same with a GUITexture? Thanks for your help.