you'd need to filter thematic_post
function kia_post($post) {
global $thematic_content_length;
if ( strtolower($thematic_content_length) == 'excerpt') {
$post = '';
$post .= get_the_excerpt();
$post = apply_filters('the_excerpt',$post);
if ( apply_filters( 'thematic_post_thumbs', TRUE) ) {
$post_title = get_the_title();
$size = apply_filters( 'thematic_post_thumb_size' , array(100,100) );
$attr = apply_filters( 'thematic_post_thumb_attr', array('title' => 'Permalink to ' . $post_title) );
if ( has_post_thumbnail() ) {
$post = '<a class="entry-thumb" href="' . get_permalink() . '" title="Permalink to ' . get_the_title() . '" >' . get_the_post_thumbnail(get_the_ID(), $size, $attr) . '</a>' . $post;
} else {
$post = '<a class="entry-thumb" href="' . get_permalink() . '" title="Permalink to ' . get_the_title() . '" ><img src="' . get_stylesheet_directory_uri(). '/images/default-thumbnail.png"/></a>' . $post;
}
}
}
return $post;
}
add_filter('thematic_post','kia_post');
untested, so beware of typos