<a class="fancybox" rel="group" href="img1.jpg"> <img src="img1_thumb.jpg"> </a>
<a class="fancybox" rel="group" href="img2.jpg"> <img src="img2_thumb.jpg"> </a>

var header = '<img id="w" src="logo.gif">';

$('.fancybox').fancybox({
    afterLoad: function() {
        this.wrap.prepend(header);
    }, // afterLoad
}); //fancybox

$('#w').click(function() {console.log('clicked');});