function getRandom(nums)
{
    var ranNum= Math.round(Math.random()*nums);
    return ranNum;
}

var numberOfImages    = 16;
var randomNumber        = getRandom(numberOfImages);

var image = new Array(numberOfImages);
image[0]="ambiance/acanthe_deco.jpg";
image[1]="ambiance/boutique.jpg";
image[2]="ambiance/camere_1.jpg";  
image[3]="ambiance/camere_2.jpg";
image[4]="ambiance/camere_3.jpg";
image[5]="ambiance/filloux_apt.jpg";
image[6]="ambiance/gd_hotel_mercure_orly.jpg";
image[7]="ambiance/salon_mo_1.jpg";
image[8]="ambiance/salon_mo_2.jpg";
image[9]="ambiance/salon_mo_3.jpg";
image[10]="ambiance/salon_mo_4.jpg";
image[11]="ambiance/salon_mo_5.jpg";
image[12]="ambiance/salon_mo_6.jpg";
image[13]="ambiance/salon_mo_7.jpg";
image[14]="ambiance/salon_mo_8.jpg";
image[15]="ambiance/tourisme_et_renovation.jpg";

document.write("<img src='" + image[randomNumber] + "' />");