Make image stretch

Put this in your style.css to make image stretch in the browser window.

body {
	background: #f2f2f2; /* Fallback for when there is no custom background color defined. */
  	-webkit-background-size: cover;
  	-moz-background-size: cover;
 	-o-background-size: cover;
  	background-size: cover;
}

https://wordpress.org/plugins/custom-background-extended/