html {
    background: linear-gradient( rgba(0, 0, 0, 0.25), rgba(0, 0, 0, 0.25) ),url(background.jpg);
    -webkit-background-size: cover;
    background-size: cover;
    background-repeat: no-repeat;
    height: 100%;
}
.logo {
    display: block;
    text-align: center;
    margin-left: auto;
    margin-right: auto;
    margin-top: 10%;
    color : #ffffff;
    font: 15px Arial, sans-serif;
}
.logo img {
    width: 20%;
    height: auto;
}

@media screen and (max-width: 1000px) {
	html{
  	background: linear-gradient(
        rgba(0, 0, 0, 0.25), rgba(0, 0, 0, 0.25)
 ),url("background2.jpg");
    -webkit-background-size: cover; /* pour anciens Chrome et Safari */
  background-size: cover; /* version standardisée */
  background-repeat: no-repeat;
}

.logo img {
    width: 100%;
    height: auto;
}
}