#header {
  background-image: url(https://preprod.arteballeto.com/wp-content/uploads/2018/06/Bandeau.png);
  background-repeat: no-repeat;
  background-position: center; 
}

@media all and (max-width: 480px) /*or whatever width you want the change to take place at*/ {
  #header {
   -webkit-background-size: cover; 
   -moz-background-size: cover; 
   -o-background-size: cover; 
   background-size: cover; 
 }
}