The items should relate in some way to the color plan and the overall mood of the room. Books and art are good starters. Books are always a welcome decor. They display the interests of the owner as well as make the room feel homey. Fine art stands alone and will always enhance a room.
Family photos are always welcome and certainly add a personal touch.
Framing is important as the art of the frame should compliment the space it shares as well as the picture it displays. Next, look to scale and balance. An oversized picture on a small wall will be uncomfortably out of place, and the opposite is also true. If only small pieces of art or pictures are available and a large wall is in need of art, arrange the small pieces decoratively on the wall to fill the space in a good aesthetic manner. The items must have some sort of relation to each other in order for this idea to work.