To reveal the House Ghosts required for the Room of Rewards, find the fallen portraits all over the Hogwarts corridors and use Wingardium Leviosa to hang them back up on the wall. Find the following portraits by searching the corresponding locations:
The Bloody Baron's portrait
In the Dungeons
The Grey Lady's portrait
Near the library on the second floor.
The Fat Friar's portrait
On the first floor, near the exit to the Stone Bridge.