function change_screenshot(app_name, add, max, ext) {
    var current_src = screenshot.src;
    
    var current_index = parseInt(current_src.match(/(\d+)/));
    var index = current_index + add;
    var newSrc = "";
    if (index > max) {
        index = 1;
    } else if (index < 1) {
        index = max;
    }
    newSrc = eval('"./img/" + app_name + "_Picture_" + index + ext');
    screenshot.src= newSrc;  
}