* {
    box-sizing: border-box;
  }

html {
    background-image: url('img/wallpaper.jpg');
    background-size: cover;
}
  [class*="col-"] {
    float: left;
    padding: 5%;
  }

img{
    border: 5px black solid;
}
.col-6 {width: 50%;}

@media only screen and (max-width: 768px) {
    /* For mobile phones: */
    [class*="col-"] {
      width: 100%;
    }
    body {
        background-image: url('img/wallpaper.jpg');
        background-size: cover;
    }
    

}