I have a slide with three images. When the slide appears, I want the
first image to be visible. When the mouse is clicked, the second image
should appear and the first should be become dimmed (or partially
transparent). When the mouse is clicked, again the third image should
appear and the second should be become dimmed (or partially
transparent).

The 'make the image appear' part is quite simple. It is the partial
fading that I cannot figure out. The images are imported .png files
with a transparent background that lets the slide's background show
through.

Any suggestions for how to accomplish this?

Thanks.