By default wordpress does not show post-thumbnails (added since WP 2.9) in home. Just in Category, Tag or Archive pages.

In the Wordpress Codex is stated that you have to implement them in your theme:
http://codex.wordpress.org/Function_...post_thumbnail

How to do that with Atahualpa? So, how to have post-thumbnails also in home?

One way to show them is by using the List Category Posts plugin and use the page you put that shortcode in as the home page. You could also use Juggledad's multi-post page template which is available to all Gold members.