In your CSS file (assuming you're using CSS because that's what you should be doing), put the following code:
body {
background: url('your_filename_here') repeat-y top center;
}
what this does is it uses the background you specify and extends it down the page in the horizontal (y) direction, this means that it will continue until (1) your content (text/picture/video) ends or (2) your pre-specified page length ends.
Replace your_filename_here with something like template.jpg, or template.png, or whatever your background file is called.