responsive background

Use this Css

#bg { width:1100px; height:400px;}
 @media only screen and (min-width: 940px)  {
#bg { width:1100px; height:400px;}
			   }
   @media only screen and (min-width: 780px) and (max-width: 939px) {
			#bg { width:770px; height:280px;}
			   }
  @media only screen and (min-width: 510px) and (max-width: 779px) {
  #bg { width:500px; height:181px;}
			   }
 @media only screen and (min-width: 0px) and (max-width: 509px) {
 #bg { width:310px; height:112px;}
			   }