Creating an image revealing effect in PowerPoint is basically a matter of layering images then using the animation tools to set the sequence in which an image is removed from a slide when it is clicked. Watch the video that is embedded below to see how that's done.
Video - How to Create Image Revealing Effects in PowerPoint
Applications for Education
Using the image revealing effect could be a good way to create a series of quiz game slides. On each slide you can have a question for which the answer is hidden until you click on the slide to reveal the answer. That could be a fun way to host an in-classroom review game that is kind of like Jeopardy.