[рони]

Poznakomlus, вариант...

function next(items) {
    var max = items.length,
        i = 0;
    return function () {
        return items[i++ % max];
    };
}

var imagName = next(['p01', 'p02', 'p03']);
alert('upload/' + imagName() + '.jpg');
alert('upload/' + imagName() + '.jpg');
alert('upload/' + imagName() + '.jpg');
alert('upload/' + imagName() + '.jpg');