Think i found a plugin for this problem.
https://www.cnet.ro/wordpress/thumbnailforexcerpts/
I also had to modify the content-extensions.php found in library/Extensions of Thematic theme folder. I realise this is not best practise for upgrades, so if anyone could tell me how to do this using my child theme funtions.php i would be very grateful.
I changed this bit:
if (is_home() || is_front_page()) {
$content = 'excerpt';
from the original:
if (is_home() || is_front_page()) {
$content = 'full';
Here is the full code
//creates the content
function thematic_content() {
if (is_home() || is_front_page()) {
$content = 'excerpt';
} elseif (is_single()) {
$content = 'full';
} elseif (is_tag()) {
$content = 'excerpt';
} elseif (is_search()) {
$content = 'excerpt';
} elseif (is_category()) {
$content = 'excerpt';
} elseif (is_author()) {
$content = 'excerpt';
} elseif (is_archive()) {
$content = 'excerpt';
}
$content = apply_filters('thematic_content', $content);
if ( strtolower($content) == 'full' ) {
$post = get_the_content(more_text());
$post = apply_filters('the_content', $post);
$post = str_replace(']]>', ']]>', $post);
} elseif ( strtolower($content) == 'excerpt') {
$post = get_the_excerpt();
} elseif ( strtolower($content) == 'none') {
} else {
$post = get_the_content(more_text());
$post = apply_filters('the_content', $post);
$post = str_replace(']]>', ']]>', $post);
}
echo apply_filters('thematic_post', $post);
} // end thematic_content