Hanging small pictures; you can do a grouping for several hangings of two or more. One thing to look for when you wonder if something works is to bring out a color say from a certain color in a throw pillow, or other accessory; the color can be in the picture, even if it is only a small splash. Subject matter doesn't always matter as much as does the color, of course discretion in subject and objects may be needed to some degree.
Once again, I will emphasize that trying to hang pictures that are directly lined up with each other is much harder than staggering them. Evenly lined can be done, especially if you don't mind extra nail holes.
Pictures should face each other, if you have two pictures each with a person in them, the people should be hung so they face one another. Balance is important, if you hang a picture of a person near the corner of the wall, you don't want the person in the painting to face the corner, put it on a wall where they are looking into the room. Any painting or picture will have a scene or object on one part that is off center; that object, the tallest one, should go toward the corner so that the room is kept in balance.