Come le dimensioni di un'immagine in base alle dimensioni dello schermo

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@media screen and (max-width: 767px){   
    #main-img{      
        content:url("img/mobile_homepage.jpg");
    } 
}

@media screen and (min-width: 768px) {      
    #main-img{      
       content:url("img/tablet_homepage.jpg");
     } 
} 
@media (min-width: 992px){      
      #main-img{        
          content:url("img/desktop_homepage.jpg");  
      } 
}
@media (min-width: 1200px) {    
     #main-img{         
         content:url("img/desktop_homepage.jpg");   
     } 
}