You don't want the words and values in there. This should work:
background:url(img/sun.png) -15px -15px;
Of course, if you don't want it to repeat, add in
Are there any other options I've missed instead of just using a bunch of empty tags? I don't need ie8 support.
CSS3 allows for multiple background on an element, so if you don't need IE8 support, go with that.