It is important that your audience "gets" the image on a slide immediately, otherwise they will be staring at it, thinking about it, trying to solve the puzzle, in short everything but return their attention to you as the speaker.
Some reasons why an image that makes perfect sense to you in your office might not work for a big audience:

  1. The clue is in the small print (as in the image used in this post). Put a big marker to draw the attention to the sentence that matters
  2. Incredibly busy compositions, Times Square in New York, a screen shot of a video game
  3. Cartoons with a lot of text. Things get worse when a small cartoon image is scaled up, leaving a poor picture quality. Give your audience time to read it, and may black out the screen to focus attention back on you
  4. (Too) interesting people: an image of the red carpet during Oscar night might make people wander off and examine those beautiful dresses rather than listen to your story about Business Process Redesign
  5. Clever graphics such as an Escher drawing. "Hey, how did he do that?"