I wouldn't make a sprite for background images, that's a world of pain. Why not just define a large background image via CSS, set it as static, vertical align to top, horizontally align to centre and allow it to clip past the window border? Then when a user resizes their browser window you'll always see the key parts of the image.
If you really want to make a 'stretchy' design, DIVs with percentage alignments are the way to go. Some advanced usage of CSS @media types will probably come in useful too so you can load a different set of image slices and have them move by different amounts depending on the initial resolution of the device; this is Responsive Design.