Show featured images from posts on main page

Hi,

I have an image slider on the main page, and it must only show 5 images from posts with featured image. (I have 33 posts, and only about 7 posts with a featured image).

At the moment I’m using this :

<?php $who_query = new WP_Query(array('cat' => '11', 'posts_per_page' => '-1')); ?>
			<?php if($who_query->have_posts()) : while($who_query->have_posts()) : $who_query->the_post(); ?>
			<?php if (has_post_thumbnail()) {
				echo '<a href="' . get_permalink($post->ID) . '" >';
				echo get_the_post_thumbnail($post->ID);
				echo '</a>';
			}
			?>
			<?php endwhile; endif;  ?>

It works fine, but it shows all 7 featured images available.

I know how I want it to work :

1. Get the loop to go through all posts
2. Get posts with featured image
3. Then show only 5 of all the featured images

But I don’t know how to get WordPress to find posts with images, but only show 5 out of those. If I change to 'posts_per_page=5, it will only grab the latest 5 posts THEN show featured images.

Is there a way to grab all posts with featured image THEN show only 5 posts?

Thanks 🙂