Read this: http://www.netbulge.com/index.php?session=0&action=read&click=open&article=1113897705 But it is generally a bad idea to do it. More advanced scripted version herr: http://kimili.com/journal/flexible-scalable-background-image But instead, get a background big enough so it always fits. Auto resizing is not a good idea and it usually looks unprofessional and bad.
You can use the theme / template: twordder It has a nice background image feature. You can tweak the rest via CSS.
I think this will help you http://www.daniweb.com/web-development/web-design/html-and-css/threads/210021