var img = document.getElementById('mainimg');
if (img != null)
    setimage(img);
function setimage(img) {
    var numberofimages = 4;
    var lastslashindex = img.src.lastIndexOf('/');
    if (lastslashindex > 0) {
        var substringed = img.src.substring(0, lastslashindex);
        var randomnumber = Math.floor(Math.random() * numberofimages);
        img.src = substringed + "/main" + (randomnumber + 1) + ".jpg";
    }
}