ReOrder Post Within Categories plugin with featured image

Since this is a plugin, it’s probably going to get axed, but generally speaking in WordPress if you want to get a posts featured image and you have access to the post ID (which I’m assuming you do), you can always do this:

$thumb_url = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );

So without looking at the plugin, it looks like you might be able to do something like this:

$thumb_url = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );
echo '<li id="' . $post->ID . '">';
echo '<img src="' . $thumb_url . '" alt="" />';
echo '<span class="title">' . $post->post_title . '</span>';
echo '</li>';

Set your styles on the image accordingly (height/width/float/margins/etc.)