From the main menu, select Croft Manor and start the level. After the load, you will be in Lara's living room.

Turn Lara on her back and face the stairs, then look on the right.

You will see a picture of a man sitting with an elm in his hand.

This is Kain, the protagonist of another Crystal Dynamics videogame, Legacy of Kain!

You can see the same picture in the level "Kain's Mausoleum" in LOK.