function setImageBg(winSize){

// get window's size
var myWidth = 1008, myHeight = 666;
if( typeof( window.innerWidth ) == 'number' ) {
	//Non-IE
	myWidth = window.innerWidth;
	myHeight = window.innerHeight;
	} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
	//IE 6+ in 'standards compliant mode'
	myWidth = document.documentElement.clientWidth;
	myHeight = document.documentElement.clientHeight;
	} else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
	//IE 4 compatible
	myWidth = document.body.clientWidth;
	myHeight = document.body.clientHeight;
}

// define image format
if(myWidth > winSize) {
	
	// locate bg image
		var urlImageBg = "images/interface/fonds/fond_body-1800.jpg";
		
	 // set background
	document.getElementById("fond-prism").style.background = "url('"+urlImageBg+"') fixed";
}
}