FB: Hoàng Thúy Quỳnh
Hello there!
To lock your iPhone with pictures in general, and iPhone 6 in particular, you need to jailbreak your iPhone first, then use the Faces tweak available on Cydia to change it. However, since you are new to this, you may feel confused. Please refer to more ways to lock iPhone with pictures to do it on your iPhone 6. Wish you success!