$.each(["img/naboutus.png","img/nwhatwedo.png","img/ntheory.png","img/nportfolio.png","img/nclients.png","img/ncontacts.png"],function(i,url){
    var img = new Image();
    img.src = url;
});

http://stackoverflow.com/questions/9197134/jquery-image-replacement-depending-on-scroll-position-only-works-properly-after

this can run before $(document).ready event, since it doesn't rely on any DOM.

preload images on jquery

jquery, images

jQuery