Actually it’s just css, not php. Adding an image behind the menucard is possible, but there are other things you need to change as well.
First of all, open the style.css file and add this to the bottom:
background: url(‘http://www.mysite.com/myimage.png’😉 50% 0;
Make sure the image is large enough. You can’t stretch background images so be sure it’s ok. This is just a start, there are other things to change as well. I’d advice to use Firebug for Firefox or Inspect Element in Chrome to pinpoint what you need to adjust.