Putting Image Slices Together

I’m considering using a theme that isn’t specifically designed for WordPress and has its bigger images in slices. From what I’ve seen on the few themes I’ve examined, the images are typically put in the background settings for a class setting, like this:

body.tree {background-image: path/oak.jpg;}

How does one use a sliced image in a WordPress theme? Do you have to put the image into the header, instead of the css file, or do you have to get a copy of the image whole? Thanks.