If the image is large eventhough i set background-size: cover; i could not see whole image for some resolution sizes code example

Example: css background image cover

html { 
  background: url(images/bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}