@media only screen and (min-width: 768px) {
   .small img{	
	  display: none;
  }   
   .big img{	
	  display: block;	  
  }  
  .jumbotron{
	background-image : url(img/intromain3.png);
	height: 55vh;	
	background-repeat:no-repeat;
	background-size:cover;
	background-position:center;
  }
}     

@media only screen and (max-width: 768px) {
  .small img {	
	  display: block;
  }   
  .big img {	
	  display: none;
  }  
  .jumbotron{
	background-image : url(img/intromain3_m.png);
	height: 55vh;	
	background-repeat:no-repeat;
	background-size:cover;
	background-position:center;
  }
}