As with text, knowing how to handle graphics properly makes all the difference. Choose graphics with care—you want images that will look good when projected onto a screen and provide something useful for you to talk about for a couple of minutes.
Keynote comes with a panoply of tools for adjusting graphics, including essential tools for resizing graphics and rotating them as well as more arcane tools for adding drop shadows and other visual effects. By and large, visual effects are best ignored unless there's a compelling reason to add them. Simply because you can add a drop shadow doesn't mean you should (to quote from Spinal Tap, "It’s such a fine line between stupid and clever").
Figure 7: On slides with a graphic of some type, simplicity works best. Adding a title might seem useful, but it doesn't really add anything you can't say out loud, and ends up reducing the space left for the graphic, causing detail to be lost (top left). Drop shadows look cool, but again, they only add visual clutter and waste space (top right). The same holds true for reflections (bottom left). Even removing the company logo frees up space allowing you to maximize the size of the graphic (bottom right).