var nb = 3;

function tourne()
{
	img1 = img2 + 1 < nb ? img2 + 1 : 0;
	img2 = img1 + 1 < nb ? img1 + 1 : 0;
	
	
	for(i = 0; i < nb; i++)
	{
		document.getElementById('part'+i).style.display = (i == img1 || i == img2) ? "block" : "none";
	}
}


var img1 = 0;
var img2 = 1;

tourne();
setInterval("tourne();", 4000);