J'ai trouvé la solution tout seul. Il faut, pour que le div fasse 100% et donc l'image aussi, mettre body et html a 100%. (d'après cet article.
Voici aussi mon css pour vous aider:
body,div,dl,dt,dd,ul,ol,li,h1,h2,h3,h4,h5,h6,pre,form,fieldset,input,textarea,p,nlockquote,th,td { margin:0; padding:0;}
table{ border-collapse:collapse;border-spacing:0;}
fieldset,img{border:0;}
address,caption,cite,code,dfn,var {font-style:normal; font-weight:normal;}
q:before,q:after{ content:'';}
abbr,acronym{border: 0;}
body{background: white repeat; font-family: Arial, Verdana, sans-serif; font-size:20px; color:black; height: 100%}
a{color:#0072bc; cursor:pointer; text-decoration:none; padding-bottom: 15px;}
html{height: 100%;}
h1{
text-align: center;
}
.slide{
background-attachment: fixed;
background-size: cover;
width: 100%;
height:100%;
position: relative;
min-height: 620px;
background-repeat: no-repeat;
}
#home{
background-image: url("images/1.jpg");
background-color: #f3f3f3;
background-size: cover;
}
#project{
background-image: url("images/2.jpg");
background-color: #f3f3f3;
background-size: cover;
}
Bon courage a tous