What separates a pretty picture from art is a message. A picture may be nice to look at but art speaks. It argues, it confronts, it demands, or it calms and soothes; depending on how the viewer perceives its message.
When making art I start by deciding what I want to say. It could be personal, trivial, social or political commentary, or even a joke. Only when I know what I want to say can I begin exploring how to say it. In a graphic design scenario the message belongs to someone else and is brought to me in a coherent (or nearly coherent) state, but it is still up to me to determine how best to express that message.
The method I return to time and again to express an idea is metaphor. Using representative imagery allows me some flexibility because it does not have to be literal. Using metaphor also helps to engage the viewer by encouraging them to discover the message for themselves. In design the metaphor needs to be more obvious than in fine art but the process remains the same.