PremieOn = new Image;
PremieOn.src = "images/btnPremieInv.jpg";
PremieOff = new Image;
PremieOff.src = "images/btnPremie.jpg";

function switchOn(img) {
                document [img].src = eval(img + "On.src");              
        }
        
function switchOff(img) {
                document [img].src = eval(img + "Off.src");                     
        }