Here's one way by using a semi transparent mask. This is best done with
photoshop or similar but it can be done in ppt

Draw a black shape over the whole picture and set the transparency to give
the faded effect you need (about 40%?).
Now add a dark blue shape where you want the highlight
Select both shapes and cut
paste special as a png
Click the blue part with the transparency wand in picture toolbar
add a fade entrance triggered by a click probably on a 99% transparent shape.

Not nearly as complex as it sounds

You could of course make several "masks" and have them appear / exit with
different clicks

Try also
Duplicate your photo and put the duplicate exactly above the former.
From this new photo cut away the parts you don't want and finally increase
its luminosity or decrease the luminosity of the former photo as well (I
don't know if you'll understand my english right now i fell sleepy!)

--
Jean-Pierre FORESTIER Microsoft MVP PowerPoint
Auteur de PowerPoint 2002 chez Micro Application