1) Nearly Headless Nick - Gryffindor

The first ghost will actually come and talk to you during the game. This is
Nearly Headless Nick and he will appear when you leave the Room of
Requirements for the first time to go and find the other DA Members. If
however you would like to find him before that then his portrait can be found
on the third floor. You will see it as you come off the Suspended Bridge next
to the Exploding Snap Champion.

2) The Grey Lady – Ravenclaw

The portrait for the Grey Lady can be found right outside the Library on the
wall opposite.

3) The Fat Friar - Hufflepuff

You will find this portrait on the first floor near the Stone Bridge. If you
come from that direction move through the round part of the corridor and it is
a little way down the next stretch of corridor.

4) The Bloody Baron – Slytherin

This portrait is hidden deep in the Dungeons of Hogwarts. When you enter the
corridor, go through the first part into the second and there will be a small
room leading off from this part. The portrait for the Baron is in there.