function preloadImages(imgs){

    var picArr = [];

    for (i = 0; i<imgs.length; i++){

        picArr[i]= new Image(100,100);
        picArr[i].src=imgs[i];


    }

}

preloadImages([
    '/theme/images/nav-act1.png',
    '/theme/images/nav-act2.png',
    '/theme/images/nav-act3.png',
    '/theme/images/nav-act4.png'
    ]);