swap image - thumbnail for large image

help please! - I have read through all the posts regarding swap images, but
can't find what I need.
I need to know how to show a large photo when a user mouseovers a thumbnail.
I know how to create a swap image, but this only allows me to show an image
that is the same size as the thumbnail - or - I can work it so that the second
image is large, but then the first image has to display with a lot of
whitespace around it, which defeats my object.
What I am trying to do is something like this...
http://www.worldofwarcraft.com/info/items/armorsets/
when you hover over the 'item stats' another text box appears. I want to do
the same sort of things as this but with a thumbnail to start with & to show a
larger photo over the top on mouseover.
I have tried several different approaches, but no good so far I'm afraid.
I appreciate any and all help. Thank you!

http://www.dynamicdrive.com/style/cs...-image-viewer/

This is exactly what I need - thank you!