You can use the the "background-size" property to resize background images:

background-size: 100px 50px /* set size to 100 x 50 pixels */
background-size: 100% 100% /* set size to 100% of the size 
                              of the containing element */
background-size: 50% /* set width to 50% of the width of 
                        the containing element and keeps 
                        the aspect ratio of the image */
background-size: auto 80px /* set height to 80 pixels and keeps 
                              the aspect ratio of the image */