In this case, we have used a combination of two images and a semi-transparent layer. The main image is copied onto the slide first. Then a box is created to precisely cover the image. By changing the colour fill of this box, you can make it a semi-transparent white, so that the image is ‘ghosted’ into the background (or ‘surbrilliant’). Finally, we then add a cropped version of the zoomed image section and add it over the top of everything so that it stands out.
The following PowerPoint tutorial video shows you how to do this (and a few other ways that it can be achieved) at about 5 minutes 30 seconds in.

http://www.m62.net/powerpoint-training/powerpoint-animation/highlighting-objects-powerpoint-2007/

Hope that this helps,

Richard