Ok. Here was my attempt at solving:
I found this line in gallery-to-slideshow-class.php:
$output = "\n\t<div class=\"gallery-to-slideshow-wrapper\">\n\t\t";
And right before it, I added:
$post_thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'medium' );
$post_thumb_url = $post_thumb['0'];
Note: you can change out medium for any thumbnail size.
I then found this line:
$featured_id = get_post_thumbnail_id( $post->ID );
And right before that, I added:
$output .= "<li><a><img src=\"$post_thumb_url\" /></a></li>";
You may have a more elegant solution, but there's my stab.