P1. A picture's function on the slide should be clear (conceptual support, graphic organizer, lucid example etc.)
P2. Pictures and illustrations should be closely related to the context of the text
The picture matches the topic of the slide, and it even helps the viewer understand the meaning of “PowerPoint”:
Cute duck, though it doesn’t necessarily match the topic:
P3. Pictures designed to organize concepts & ideas should incorporate devices stressing temporal (time), conceptual, and/or spatial relationships, such as Storyboard layout; Sequence emphasized by arrows, numbers, or labels; Pictures in sequence presented one at a time in correct sequence (or video used to achieve same result); Spatial organization utilizes 3-D diagrams or superimposition of features.
Here is a good examples of a sequence presented in flowchart fashion:
P4. Pad the space around graphics (don’t let text get too close to your pictures)