CSS Background Images -- Fixed or Scrolling
As part of the "background" value, scrolling is the default behavior of an image set as a CSS background. Scrolling, as may be obvious, suggests the image will scroll away... right off the page, even -- goodness! Or this is certainly possible when the page content is of sufficient length to allow mouse-wheels or other user-initiated movement to cause vertical scrolling.
The option here is to instead choose fixed as your background image modifier. In this case, the image differs only in that it does not scroll away -- but instead remains stubbornly in its original position. Used properly, this creates the illusion that your page content is moving while the background image you've selected... is not.
Of course, you'll ruin this with a picture of your cat's prize-winning hairball -- ho ho, up and down the page it skitters! But trust the herd on this much... a fixed background, used creatively, can generate appreciative whistles... even among blog snobs who talk utter clap-trap and say feng shui no less than twice an hour.
As a treat for reading this, you may impress your snooty mates by noting that feng shui roughly comes to "wind-water" -- which is to say, nothing important. Break one, drink the other. Still, say it loud enough, and you'll shine among a bleating slew of your peers munching at cocktail olives. Remember it all the same.