A good use of animation is to show change or contrast. For example, you can make a winter scene disappear as a summer scene appears, to show the difference. At the bottom of this tip, I have some links to other tips on animation.
In previous versions of PowerPoint the fade transition didn’t look very good, so most people put both photos on one slide and faded the first out as the second came in. This still works well.
It’s worthwhile to try out all the animations to see what they do, especially with their variations. You never know when one will give you just the effect you want.
But PowerPoint 2010 has added some new transitions, including some 3D ones, that look much better. The 3D transitions let you watch the old slide exit as the new one enters, and so helps connect the two slides.
In this video, the first effect is a Split exit animation on a photo, which reveals a second photo beneath it. The second effect is one of the new 3D PowerPoint 2010 transitions, called Rotate.