var valueURL = "imgs/value1.jpg";

if (document.images) {
     var value1 = new Image();
     value1.src = "imgs/value1.jpg";

     var value2 = new Image();
     value2.src = "imgs/value2.jpg";
}

function changeValue() {
     if (document.images) {
        if (valueURL == "imgs/value1.jpg") valueURL = "imgs/value2.jpg";
		
         document.myValue.src = valueURL;
	}
}