I'd guess that two versions of the same photo have been inserted in the same
slide: one in b/w and one in color. The color photo then covers up the b/w
version with the animation "Fade in" set to very slow.
In Edit view try to move the color photo to the side to see whether
something is underneath it, show the custom animation task pane to look
which animations have been used.
You are right. It works like a charm and makes a nice effect. I don't know
enough about PowerPoint to have ever thought of that process. Thanks a
million.