<body>
  <div id='main'>
    <div id='row0'>
      <img src='cards/back.png' class='card'>
      ...
      <img src='cards/back.png' class='card'>
    </div>
    <img id='join' src='images/join.png'>
  </div>
</body>
#main {
  display:flex;
  flex-direction:column;
  align-items:center;
}
#join {
  height:8vmin;
}