body {
  display:flex;
  align-items: center;
  justify-content:center;
  height: 100vh;
  background-image: url(bg.jpg);
  background-position: center center;
  background-size: cover;
}
body::before{
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: black;
  opacity: 0.3;
}

.landingspage {
  position: relative;
  z-index: 10;
  display:flex;
  width:80%;
  height: 85%;
  align-items:center;
  justify-content: center;
}
.landingspage img{
  width:100%;
  height: 100%;
  object-fit: contain;
}

.landingspage img.small{
  display: none;
}

@media only screen and (max-width: 800px) {
  .landingspage{
    width: 90%;
    height: 90%;
  }
  .landingspage img.big{
    display: none;
  }
  .landingspage img.small{
    display: block;
  }
}