<div id="slogan">
                         <script type="text/javascript"> 
if (screen.width > 768){
    document.write("<h3><img src='img/header.png' alt='' /></h3>") ;    
        
}
else
{
  document.write("<h3><img src='img/header-mobile.png' alt='' /></h3>") ;
}
       </script>      
                           

                 </div>

<script type="text/javascript">
	var EcranLarg = screen.width;
	if((EcranLarg < 1300)) {
	   document.write('<link href="css/style2.css" media="screen" rel="stylesheet" type="text/css" />') }
	else{
	   document.write('<link href="css/style.css" media="screen" rel="stylesheet" type="text/css" />') }
	</script>

<div id="slogan">
        <script type="text/javascript">    
           var windowWidth = $(window).width(); 
                if (windowWidth > 800){
                     document.write("<h3><img src='img/header.png' alt=''/></h3>") ;    
                      }    

               else{
                   document.write("<h3><img src='img/header-mobile.png' alt=''/></h3>") ;    
                  }

       </script>      
</div>

#id nav:after {
content: url(chemin/img.png);
}

if(screen.width>640) {
   var img_list = document.getElementsByTagName('img');
   var i = 0;
   while (element = img_list[i++]) {
      if (element.className == 'kiwi') {
      var url = element.src;
      var posUrl = url.lastIndexOf('_');
      var preUrl = url.substring(0,(posUrl+1));
      var newUrl = preUrl+'big.jpg';
      element.src = newUrl;
     }
   }
}