#slide1 { background: url(/CSS/gateau.png) center 0 no-repeat fixed; background-size: cover; width:100%; height:712px;}
#slide2 { background: url(/CSS/test.png) center 0 no-repeat fixed; background-size: cover; width:100%; height:712px;}
#slide3 { background: url(/CSS/gateau.png) center 0 no-repeat fixed; background-size: cover; width:100%; height:712px;}

.slide_inside { padding: 50px;  }


<div id="slide1"><div class="slide_inside"></div></div>
<div id="slide2"><div class="slide_inside"></div></div>
<div id="slide3"><div class="slide_inside"></div></div>