body, html {
  height: 100%;
  margin: 0;
}

.center-container {
  display: flex;
  justify-content: center;  
  align-items: center;      
  height: 100vh;            

  /* Responsive minimums */
  min-width: 60vw;   /* 60% of viewport width */
  min-height: 70vh;  /* 70% of viewport height */
}

.center-container img {
  max-width: 80%;   /* image shrinks on small screens */
  height: auto;
}