As thirtydot said, you can use the CSS3 background-size
syntax:
For example:
-o-background-size:35% auto;
-webkit-background-size:35% auto;
-moz-background-size:35% auto;
background-size:35% auto;
However, as also stated by thirtydot, this does not work in IE6, 7 and 8.
See the following links for more information about background-size
:
http://www.w3.org/TR/css3-background/#the-background-size