thumbnail problem with Uncomplicated theme

Hello! This is my forst experience with WordPress.
I am trying to use the theme Uncomplicated downloaded at this link:
http://www.smashingmagazine.com/2010/08/19/100-free-high-quality-wordpress-themes-for-2010/

I succeded in installing and using this theme but I cannot
managed to visualize the thumbnails at right column in the first page of my blog.
( I used the spefic button to post ” image thumbnail” for my article but I think I have to change something in the code of the theme)

Please can you help me with the code? Which file I have to change?
thank you very much
Violante