It's just the (in)appropriate match between head and bodyin this case, the very familiar image Rob's winning smile atop a skeleton waving a sword around with a great deal of enthusiasm and more than a little campness. It's not really a joke to think about, it just looks daft.
(
Photoshop Bitch 2014 edition,
Sat 7 Apr 2012, 21:45,
archived)