Instead of using a fixed border, is it possible to set an image as the border by editing the style.css
Yep. Set the image as a repeating background on the container in question, and add padding to that container to accommodate the image you're repeating. If you need to do this on all four sides, you might need to nest four containers to do it, and then apply all four sides of padding to the innermost container. When CSS3 is approved, released and supported, you'll be able to attack multiple backgrounds to a single object. For now, nesting is necessary.