Jon

#block-system-main-menu .menu,
#block-system-main-menu .menu li {
  margin: 0;
  padding: 0;
  list-style: none;
}
  #block-system-main-menu .menu li {
    display: block;
    overflow: hidden;
    clear: both;
  }
    #block-system-main-menu .menu li a {
      display: block;
      float: left;
      width: 100px;
      padding: 20px 0;
      background: #494949;
      margin: 2px;
      text-align: center;
      color: #fff;
      text-decoration: none;
    }

     #block-system-main-menu .menu li:nth-child(1) a {
   background: url(../images/home.png);
  }
  #block-system-main-menu .menu li:nth-child(2) a {
    background: url(../images/aboutus.png);
  }
  #block-system-main-menu .menu li:nth-child(3) a {
     background: url(../images/products.png);
  }

  #block-system-main-menu .menu li:nth-child(4) a {
     background: url(../images/contactus.png);
  }