I do not think that the organ does anything.

However, when you ring the bell in Ultimecia castle, a countdown appears. You must ring the bell, then switch to your other party. The other party must be in the room with the fountain in it and you should have the ability which allows you not to encounter any monsters as that helps. Run into the room with the organ. If you made it in time - the swirling dark matter in the room will be replaced with the omega weapon.

To easily defeat him, use heroes and holy wars to make you invincible, while using limit breaks.

And finally for the pictures - you must select the sleep picture, the garden picture, and the messenger picture. I forgot the actual names of them but you can check them yourself by looking at the other pictures in the room.

Sucessfully doing so will release the fairly easy trauma boss who has sealed one of your powers. Find your own method of defeating him.