#m-doboz {
    background-color:blue;
    position:relative;
    width:300px; 
    height:300px;}
#m-doboz .top, #m-doboz .bottom{
    background-size:cover;
    position:absolute;
    top:0;
    left:0;
    width:100%;height:100%;
      -webkit-transition: opacity 1s ease-in-out;
  -moz-transition: opacity 1s ease-in-out;
  -o-transition: opacity 1s ease-in-out;
  transition: opacity 1s ease-in-out;
}
#m-doboz .top {
    left:30px;top:30px;
    background-image:url(aktiv-b1.png);
    background-size:cover;
    }
#m-doboz .bottom {
    opacity:0;
    background-image:url(aktiv-c1.png);
    }
#m-doboz .top:hover{opacity:0;}
#m-doboz .bottom:hover{opacity:1;}