The page you are looking at can be done with simple HTML, and probably is on the National Geographic site.
A Basic way to do this is:
Create an HTML table that has a number of rows equal to the number of thumbs you want to show, and 2 columns.
Make the second column first row extend over all of the other rows.
Load your thumbs into column 1 of the rows you've created.
Load your full size image into column 2 row 1.
Those are the basics, now all you need to do is write the code that displays the various pieces you want to display.
