Tutorial Basic Link Rollover as CSS Sprite
a {
display: block;
background: url(sprite.png) no-repeat;
height: 30px;
width: 250px;
}
a:hover {
background-position: 0 -30px;
}
The set height and width ensure only a portion of the sprite.png graphic is shown. The rollover shifts the position of the background image, revealing a different area of the graphic.