Just so you know the top one was actually a winner in a photoshop contest. Not sure about the other ones but I have my doubts about their legitimacy. Heres why.
The book of Enoch said that they were incredibly violent. So violent the mothers died in labour and they enjoyed not only killing people but crushing and eating them. When God wanted them all gone, the book of Enoch says Saint Gabriel gathered all the nephilim together in one place to kill each other in mutual slaughter. Not one remained.
Assuming how violent they were I highly doubt you would find a single intact skeleton of one, if you found any trace of them.