Adapter la largeur d'une image à la taille de l'écran

	<div id="retourmenu">
		<img src=""quot;images/retourmenu.jpg" alt="" onclick="javascript:menu();" ><span onclick="javascript:menu();">Menu</span>
	</div>

	<div id="mainpage">
		<div id="menu"><br />
			<nav>
			<ul>
				<li><a href="index.php?page=accueil" rel="nofollow noopener noreferrer" target="_blank">Accueil</a></li>
			</ul>
			</nav>
		</div>
		<div id="contenu" class="clearfix">
</div>

function menu(){
	document.getElementById('menu').style.width = "100%";
	document.getElementById('menu').style.textAlign = "center";
	document.getElementById('menu').style.fontSize = "1.3em";
	document.getElementById('menu').style.backgroundImage = "url(images/parchemin2.jpg)";
	document.getElementById('menu').style.backgroundSize = "100%";
	document.getElementById('menu').style.backgroundRepeat = "no-repeat";
	document.getElementById('menu').style.display = "inline";
	document.getElementById('contenu').style.width = "0%";
	document.getElementById('contenu').style.display = "none";
}