"The one we used at the other end of our tunnel," Neville explained. "Aberforth's picture led to another one at Hogwarts that created the link. When the Room of Requirement opened the tunnel for us, the portrait served as our gateway, but it had to be taken from another part of the castle where it usually lived. Luckily for us, it had been moved from it's normal position so the Carrows and other Death Eaters didn't notice it was missing."