Wouldn’t an interesting image like this grab your audience’s attention? 

I certainly think so.  According to this article about effective PowerPoint presentations, clip art is the devil and should be avoided at all costs.  I agree.  Some of the best presentations I have seen included images that got me thinking, “Where on earth did they find that great picture!”  When the captured scene is the perfect illustration for the speaker’s ideas, it creates a memorable experience for the viewer.  I’m still learning the art of locating great images, but ImageChef is an interesting one for customizing graphics (like the one above).