title: Adjust background size for an advertising skin
body: <p>I'm trying to fix an issue. I have a background advertising (skin) and I must increase the size: the same adv is served for two website, but one of the website has a larger container.</p>
  
  <p>I try with background-size property, but when I try to set "110%" the behavior of the background is related to the windows, so if I resize the browser the skin follow the browser.</p>
  
  <p>Ho can I increase the size of the background keeping it centered on the container?</p>