The rule of thirds
This concept guides you on how to position subject images in a picture; I actually find it very important when it comes to photography and similar types of imagery art.
In the example I have used it shows a tree as the main subject of the picture, placing it in a suggested point based in the rule of thirds.
Example:
Place the most important part of your picture--the center of interest--at one of the four places where the imaginary lines cross. A person who is the subject of interest should look or appear to be moving toward the center of the picture. When taking a picture with a horizon, place the horizon line on one of the horizontal thirds, depending on the emphasis you want in the picture.
When taking photos of people, don't chop them off at the knees or ankles. Include the entire body or shoot your subject from the waist up.