moskitos80,
на всякий случай ...
$('.sea_content img').replaceWith(function(){
var thumb = this.src,
fullImage = thumb.replace(/\/thumbs/, "");
return '<a class="fancybox" href="' + fullImage + '" data-fancybox-group="gallery" title="">' +
'<img class="fancybox_img" src="' + thumb + '" width="200" alt="poseidon">' +
'</a>';
});