$(function() {
    var image = function(i) {
        return '/static/img/header' + i + '.jpg';
    };
    var elem = $('.header .main');
    var currImage = 1;
    elem.css('backgroundImage', 'url(' + image(1) + ')');

    setInterval(function() {
        currImage = currImage % 6 + 1;  // counts repeatedly from 1 to 6
        elem.BgImageTransition(image(currImage));
    }, 15000);
})