html { 
    background: url(achtergrond.jpg) no-repeat top fixed; 
    background-size: cover;
  }
.main {
    margin: auto;
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
}
.een img {
    margin-top: 130px;
    width: 50vw;
}
.twee img {
    margin-top: 70px;
    width: 40vw;
}