Hmm, I deleted the theme... let me ask a few Q's before I reinstall it.
So, we're on the same page that this theme does not display post thumbnails on the main page (take a look at the code for the index.php and you'll see it displays only the excerpt)?
"no image" sounds familiar, let's see: Hamachi/index.php line 19:
<?php if($image_src != '') { echo $image_src; } else { ?><?php bloginfo('template_directory'); ?>/images/no-img.gif
So if the code searches the post ($content) and does not find an img
tag it will display the default "no image" image found here: themes/hamachi/images/no-img.gif
If you wanted to display a different image in that case (no image tag found in $content) then you would replace the file no-img.gif with the one you want to use (using the same name and dimensions), but I suspect you know that...
Hold on, let me install it and check something...