title: Give NSWindow a background image
body: <p>Ok, so I've created an image in Photoshop that will align with the buttons on my app, and now I'd like to make it the background image of my window so that the characters on the image will correspond to the keys on my app (a small calculator demo app I've been working on)</p>
  
  <p>Basically, instead of giving buttons Text like 1,2,3,4, etc. I've made a 3x3 map with numbers of a different font just because it will look pretty</p>
  
  <p>What I'm having difficulty with now is that I can't seem to make the image the background of my window.</p>
  
  <p>I created an NSImageView and I dragged the image file onto it, so I can see it now, but I can't make it the background.</p>
  
  <p>Do I need to subclass the NSImageView or is there some simple method?</p>
  
  <p>I'm using XCode 4, btw</p>
  
  <p>Thanks!</p>