[Plugin: Latest Posts by Author] show post thumbnail

Create a new line just after this line in the plugin (I think it should be line 24):

$html .= '<li><a href="'.$numpost->guid.'">'.substr($numpost->post_title, 0, 10).'</a>';

In the blank line you just created, place the following code to display the thumbnail:

$html .= get_the_post_thumbnail($numpost->ID);