Miramar - You want all the small thumbnail images to be the same shape. Is this correct. But you want the pop up version to show the entire image. Right? Like this? http://www.placitasartists.com/t_perkins/

If this is what you want, then you have to create a new layer for each full size image. Paste each image on its own layer.

Link each thumbnail to the popup layer that has the full image.

You can use the Link > Popup Layer which shows the layer when the visitor clicks the thumbnail image
or Website Properties > Mouse-Over > Show Pop-up Layer which shows the layer when your visitor mouses over the thumbnail.