.content{
    margin:auto;
    width: 50%;
}
.small{
    width: 250x; 
    height: 181px;
}
.opacity-on{
    opacity : .5;
}
.opacity-off{
    opacity:1.0;
}
img:hover{
    opacity:1.0;
}
.main-picture{
    margin: 0 auto;
}