[Plugin: Multiple Featured Images] Can't get this plugin to work with my theme..

The theme uses a posts featured image in both a slider and in the loop. The two instances require different sizes (not the same aspect ratio). I can get one working by uploading images of the right size, but then the other appears stretched, and visa versa. I thought the solution might be to use your/this plugin, but I can’t work out how to integrate it with existing code.

Here are the relevant bits.

Theme functions for extra image sizes/thumbnails:

function sight_setup() {
	update_option('thumbnail_size_w', 290);
    update_option('thumbnail_size_h', 290);
    add_image_size( 'mini-thumbnail', 50, 50, true );
    add_image_size( 'slide', 640, auto, true );
    register_nav_menu('Navigation', __('Navigation'));
    register_nav_menu('Top menu', __('Top menu'));
}

loop code:

<div <?php post_class('post clear'); ?> id="post_<?php the_ID(); ?>">
            <?php if ( has_post_thumbnail() ) :?>
            <a href="<?php the_permalink() ?>" class="thumb"><?php the_post_thumbnail('thumbnail', array(
                        'alt'	=> trim(strip_tags( $post->post_title )),
                        'title'	=> trim(strip_tags( $post->post_title )),
                    )); ?></a>
            <?php endif; ?>

slideshow code:

<?php if ( has_post_thumbnail() ) echo '<a href="'.get_permalink().'">'.get_the_post_thumbnail($post->ID, 'slide',
                    array(
                        'alt'	=> trim(strip_tags( $post->post_title )),
                        'title'	=> trim(strip_tags( $post->post_title )),
                    )).'</a>'; ?>
                <div class="post-category"><?php the_category(' / '); ?></div>

Hope somebody can help!
Kaspar
PS. Our work in progress is here: https://stuttg-arttours.de/ (currently stretching the square thumbnails used in the loop)