Heres is a link to explain skull theory - http://genderdreaming.com/forum/ultrasound-gender-prediction/6591-skull-shape-theory-early-gender-determination.html
Skull theory is more purely a fun guess, whereas the nub theory is much more accurate. I have had a another peak at pic but I still cant make out the nub, sorry. Im still new to this though so hoepfully someone with more experience can take a look too x