<!DOCTYPE html>
<html>
<head>
  <title>background-size: contain; reference</title>
  <style type="text/css">
#outer
{
  border: 10px solid black;
  width: 64px;
  height: 128px;
}
#inner
{
  width: 64px;
  height: 64px;
  background-image: url(aqua-32x32.png);
}
  </style>
</head>
<body>
<div id="outer"><div id="inner"></div></div>
</body>
</html>