Some might consider it a cliché, but I found it still useful: the school of fish swimming in formation to create the illusion of being a shark. For when you need to visualize how many smaller/weaker entities can work together to become very strong as a group.
An image like this can easily be created by searching for "fish silhouette" or "shark silhouette" in a stock photo site. Resize the small fish, paste them over the shark's silhouette, and off you go.
Inspired by a scene from the movie Finding Nemo:
UPDATE: I have now added a slide with many fish forming a shark on this concept in the SlideMagic template store.