div.main {
  width: 50%;
  margin-left: auto;
  margin-right: auto;
  display: block;
  float: none;
}

.main img {
  height: 100%;
  width: 100%;
  display: block;
}

body {
  min-height: 100vh;
  margin: 0;
  display: flex; 
  justify-content: center; 
  align-items: center;
}

@media (max-width: 600px) {
    div.main {
        width: 90%;
        margin: 0 auto;
    }
}