(540 WFLA) In the DVD release for HBO's Game of Thrones, episode 10, show-runners Dave Benioff and D.B. Weiss admit to using the severed head of former president George W. Bush in the scene below. The show-runners statement follows.
"The last head on the left is George Bush. George Bush’s head appears in a couple of beheading scenes. It’s not a choice, it’s not a political statement. We just had to use whatever head we had lying around." - Dave Benioff & D.B. Weiss
You can see the footage at the 1:10 mark in this video:
Here's a still frame of the shot in question: