Look at the bottom of the Photo Slideshows Widgets folder. There are several Zoom widgets that let you mouse over an area and display a larger version.
You can also use pop up layers to display larger versions of images when the visitor mouses over certain images. See this page http://www.gwpriester.com/Logo-Design.html
Each larger logo is displayed on a pop up layer. The thumbnails link to the pop up layers via a Website Properties > Mouse-over > Show Pop-up layer.
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