var imageList = ["tree2", "tree3", "tree4", "tree5"];
var selected = 0;

function showImage(id, visible) {
    var element = document.getElementById(id);
    if (element) {
        element.setAttribute("style", visible ? "" : "display:none");
    }
}

function nextImage() {
    showImage(imageList[selected], false);
    selected++;
    if (selected == imageList.length) {
        selected = 0;
    }
    
    showImage(imageList[selected], true);
    setTimeout("nextImage()", 3000);
}

setTimeout("nextImage()", 3000);