eclev91

This plugin introduces a couple of nifty functions to replace <code>the_post_thumbnail()</code> & <code>get_the_post_thumbnail()</code> in exchange for the responsive <code>picture</code> element. Use:
<pre><code>get_the_post_picture(
 $fallback_image_size,
 array(
 $breakpoint_1=>$image_size_1,
 $breakpoint_2=>$image_size_2
 )
 [, $id, $atts]
);
</code></pre>
For example:
<pre><code><?php echo get_the_post_picture(
 'single-featured-sm',
 array(
 '(min-width:768px)'=>'single-featured-md',
 '(min-width:992px)'=>'single-featured-lg',
 '(min-width:1200px)'=>'single-featured'
 )
); ?>
</code></pre>
While this says 4.0 for minimum version, I would wager it’ll be fine on most older installations.

Picture Element Thumbnails

Get a picture element instead of an image tag for responsive images on your WordPress site.

voceplatforms

Chris Scott

Adds multiple post thumbnails to a post type. If you’ve ever wanted more than one Featured Image on a post, this plugin is for you.

Multiple Post Thumbnails

Even though this plugin hasn't been updated in a long time, it still works great! So I guess there's just no reason for the developer to have it updated. ;-)
For me this plugin is one of the easiest ways to selectively add a header image. I want it on some pages, but not on others. Using the standard Featured Image function will set it foor all or none, so that's not flexible enough.
One suggestion for an update though:
Now you still need to add the following snippet to your theme's function.php.
if (class_exists('MultiPostThumbnails')) {
    new MultiPostThumbnails(
        array(
            'label' => 'Secondary Image',
            'id' => 'secondary-image',
            'post_type' => 'post'
            'post_type' => 'page'
        )
    );
}
It would be great if the plugin did this automatically. That will also keep the plugin active if you switch themes. 
P.S.
To add the meta box to other post types, just add it to the snippet.
'post_type' => 'event' will add it for Events Manager, for example. ;-)



Still works great!

Adds multiple post thumbnails to a post type. If you've ever wanted more than one Featured Image on a post, this plugin is for you.

Picture Element Thumbnails vs Multiple Post Thumbnails