A header gallery with different image sizes based on position to reduce file size.

header-gallery.php

<? php
// Don't include opening php tag


  // Cmb2 meta feild prefix
  $prefix = '_homepage_header_gallery_'
  
  // Get the list of files
	$files = get_post_meta( get_the_ID(), $prefix . 'images', 1 );

	echo '<div class="header-image-collage-wrap">';
	// Loop through them and output an image

	$counter = 0;

	foreach ( (array) $files as $attachment_id => $attachment_url ) {

		$full = array( 0, 3 );
		$half = array( 1, 2, 4, 5, 6, 7 );

		if ( in_array( $counter, $full ) ) {
			$image_size_name = 'header-image';
			$img_size = 'header-image';
		} elseif ( in_array( $counter, $half ) ) {
			$image_size_name = 'medium';
			$img_size = 'medium';
		}

		echo '<div class="collage-image image-size_'.$image_size_name.'">';
		echo wp_get_attachment_image( $attachment_id, $img_size );
		echo '</div>';

		$counter++;

	}
	echo '</div>';