html, body {
  height: 100%;
  background: #ccc;
  padding: 20px;
}
body {
  padding: 20px;
  background: white url(https://www.webhek.com/demos/codepens/robot-dog.webp) calc(100% - 25px) calc(100% - 25px) no-repeat;
  
}
* {
  box-sizing: border-box;
}