Create a Hyperlink on the image that links to the full image. When you
preview the page in the browser and click on the thumb nail, the full image
should appear.
If you are using an FP generated Gallery, provided that you have imprted the
pictures into your "Images" folder and not just saved them, they should
automatically open on a single click.
If not then Singapore Web Designs answer would apply.
HTH
Teriel9