#logo {
  background: none;
  padding-left: 4px;
}
#logo a {
  background-image: url(logos/2015/pa-penntrackxc.png);
  background-size: contain;
  background-position: center center;
  background-repeat: no-repeat;
}
body.scrolled #logo a {
  background-image: url(logos/2015/pa-penntrackxc2.png);
}