"Pictures are understood on many levels. The most literal level is what the picture depicts. When you see a line drawing of an airplane, you recognize the shape and features of the object and identify it as an airplane.

“On another level, the context of the picture provides meaning. The same picture of an airplane on a freeway sign means that an upcoming exit will take you to the airport. This is a different context than a photograph of an airplane you may see in an airline advertisement, which suggests that is is persuasive rather than an informational purpose.

“Understanding the meaning of the picture depends on the context of where the picture exists. Another level of meaning is based on the style of the graphic. This is expressed in many ways, such as through symbols, spatial layout, and accepted conventions. For example, certain attributes of an illustration indicate when a drawing is an architectural blueprint and when it is a scientific illustration.

“There are also metaphoric meanings in some graphic. Metaphors convey meaning beyond a simple depiction and provide another layer of meaning."

Excerpted from Connie Malamed's Visual Design Solutions - a fantastic text for understanding the power of using visuals in learning.