completely wordpress native image "zoom"

I’ve got a real challenge for you guys.
I am writing a mod for a plugin and I don’t want to depend on other plugins, and be as lightweight as possible.

The function I am looking for is as simple as when the user mouses over an image for the full size image to show up on screen next to or above the smaller image. I would like to do this using only what is available in wordpress and not having to bundle any js files. Extra js may be ok if it is hosted like jquery is by google.

Thanks for any help