It seems a lot of people didn't get the ending. Maybe this will help:
- George Claire supposedly wrote his dissertation on George Innes, a famous American landscape artist. Innes was a follower of Swedenborg, who believed (among other things) there is a connection to Heaven and Hell through nature. The movie was sprinkled with gorgeous landscape views, with dramatic lighting and brilliant colors, meant to evoke Inness' paintings.
- Floyd's first meeting with George includes a reference to Swedenborg, and Floyd holds up his book with an Innes painting depicting a a cross in the sky, meant to be a portal to Heaven. He states he is a follower in Swedenborg's beliefs, believes in the existence of spirits who will help those in the physical world, and sees nature's connection to spiritualism in Inness' paintings.
- Cathy and Floyd see the female spirits during the seance. They are being held back by a malevolent male spirit.
- After George murders Floyd, the spirits confront George with his crime using George Inness' painting with the cross during the classroom slideshow. This painting is also a metaphor for Floyd's death in the water; Floyd has gone to Heaven, and has, in fact, become a re-enactment of his favorite Inness painting.
- The female spirits of the house (seen earlier in the seance) try to help Cathy, but cannot do much in the face of a man's free will (I.e. George's decision to murder Cathy). What they can do to help is wake up Justine.
- With Justine's threat becoming a reality, George tries to flee on the boat, however the spirits confront him again, using the topic of his supposed dissertation. But the Inness painting has changed - this time the cross is upside down, and a fiery portal is opening up under the waves. George is going to Hell (either literally or figuratively), and has now become the subject of an Inness painting that is the *reverse* of Floyd's painting.
- The painting depicting George's hellacious fate hangs on a wall. We see through the flash of the woman's ring that it is across from the photo of the Smit family who also became the spirits in the household. The female spirits had successfully avenged Catherine, thus finally taking care of one of their own.