Not every image that is used in advertising has a functional objective. Take fashion ads, for example, sometimes the product is missing all together. 

  1. Images that show something highly specific: a product, a medical condition, a location
  2. Images that show a relevant scene or background: people tapping on their mobile phone, a driver in a traffic jam, calm bamboo forest, a sunset
  3. Images that make a visual metaphor: a prisoner in a cage, a cat chasing a mouse
  4. Images that just set the mood of the presentation

I use 1. and 4. more, and 2. and 3. less because they often lead to visual cliches. 

Art: "Self-Portrait, Yawning" by Joseph Ducreux, 1783