"sensitising" an image

[TheKedge]

Hello all,
I've got a big image that I'd like to use as the UI in an application. A small part of the thing is attached.

What's the best way of senseitising it?
Ideally, I'd like to click on the image somewhere and change that small part of the image (by covering it with another smaller image OR with a small widget - giving the appearance of something changing state).

I assume I'll have to cover all of my "active" regions with widgets or images. which is best?
What's best for tracking the active areas if the user resizes the top widget? Should I just scale the original "measured" map, or should I go looking for things in the original image?

Or should I make a "farm" of widgets instead of using the original image?
What's the best way of doing this? any ideas?

Thanks
K