Rectangles:

Most artwork or portraits are created in the rectangle format because they are easy to construct, frame and display and work well for most subject matter. Consequently, rectangles are the most common type of frame. 

Horizontal vs. Vertical Orientation:

Orientation affects how the subject is perceived. A vertical subject such as a standing figure, tree, or mast of a ship appears taller in the upright format. Similarly, the horizon line of a landscape is accentuated by a horizontal format.

Tip 1: Choose a rectangle frame with a vertical orientation if you want to emphasize a portrait with a taller object such as a tree or a standing woman or man.

Tip 2: Choose a rectangle frame with a horizontal orientation if you want to emphasize elements in a portrait that have that same horizontal orientation, such as a boat on water, or a sunset.

Squares:

Squares lend themselves to symmetrical compositions of two types: elements perfectly centered or elements that rotate around the center. The center of a square is the focus and will exert a gravitational pull on the elements around it.

Tip 3: Use square frames for abstract artwork to emphasize the depth of the art.

Rounds:

Because it has no beginning or end, the circle is a universal symbol for infinity and completeness. As a result, a round frame is perfectly suited for representations of the ideal and everlasting. Because the geometry of the circle always leads us back to the center, anything placed there will take on a significance that it wouldn't have in another location. Thus, the object will be will be greatly accentuated.

Tip 4: Choose a round frame for artwork or portraits that have special significance or meaning to you, and whose objects are centered.

Oval:

An oval has the characteristics of both the circle and the rectangle. Upright, the oval frame emphasizes the elements closest to the top and bottom curves.

Tip 5: Oval frames look great with classic portraits. They do a great job at emphasizing the face or faces of those pictured.  Flower arrangements also look great in oval frames, because flowers echo the curves of the oval frame.