body {
  margin: 0;
  background-image: url("fondoverde1.jpg");
  background-size: 100%;
  color: #999;
  display: flex;
  flex-wrap: wrap;
}
img{
  width: 200px;
  margin: 10px;
}
@media (max-width:768px){
  body{
    background-image: url("fondoverde2.jpg");
  }
}