.slideshow {
   width: 150px;
   height: 120px;
   overflow: hidden;
   border: 3px solid #F2F2F2;
   margin: auto;
}

.slideshow ul {
    /* 4 images donc 4 x 100% */
   width: 400%;
   height: 200px;
   padding:0; margin:0;
   list-style: none;
}
.slideshow li {
   float: left;
}