var images = ['image-1.jpg','image-2.jpg','image-3.jpg','image-4.jpg','image-5.jpg','image-6.jpg'],
   index = 0,
   maxImages = images.length - 1;
var timer = setInterval(function() {
   var curImage = images[index];
   index = (index == maxImages) ? 0 : ++index;
   $('.header').css({
    'background-image': 'url('+curImage+')',
    'transition': 'background 500ms ease'
   });
}, 1500);