Feature Request: Featured Image

Its not a big thing as you think if you use template tag. Just add
<a href="<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?></a>
and use css to show the image the way you want.
to change the thumbnail size check this link Click here

the full code

<?php
echo '<ul>';
$posts = wmp_get_popular( array( 'limit' => 10, 'post_type' => 'post', 'range' => 'monthly' ) );
global $post;
if ( count( $posts ) > 0 ): foreach ( $posts as $post ):
    setup_postdata( $post );
    ?>
    <li><a href="<?php the_permalink(); ?>"><?php the_post_thumbnail('your_custom_thumbnail'); ?></a><a href="<?php the_permalink() ?>" title="<?php echo esc_attr(get_the_title() ? get_the_title() : get_the_ID()); ?>"><?php if ( get_the_title() ) the_title(); else the_ID(); ?></a></li>

    <?php
endforeach; endif;
echo '</ul>';
?>