Zoomed in section of an image

I want to show an image (actually, a page in a document) on the slide and when leaners roll over it they see a zoomed in view of that section of the image.  I thought there was a tutorial that showed you could have a large size version and a small size version of the same image and create a shape that looked kind of like a magnifier.  Does anyone recall that tutorial?  I thought I saw a posting on this somewhere on this site but despite hours of searching I can’t find it.

Thanks for your help. 

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