WordPress get post thumbnails in a custom pattern

Question

Right now I have for each post two sizes for thumbnail:

$big = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_600x200' );
$small = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_200x100' );

What I am trying to achieve is to display posts using this next pattern:

Post 1 - [big thumbail]
Post 2 - [small thumbail]
Post 3 - [small thumbail]
Post 4 - [big thumbail]
Post 5 - [small thumbail]
Post 6 - [small thumbail]

Actually posts will be shown big - small - small - big - small -small and so on.

Any idea? Thank you

This is my post foreach:

<?php foreach ($posts as $post) { 
    $big = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_600x200' );
    $small = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_200x100' );

    if ( $big ) { ?>
        <img src="<?php echo $big['0']; ?>" />
    <?php }else{ ?>
        <img src="http://placehold.it/600x200/7f8c8d/ffffff" alt="Featured image missing"/>
    <?php } ?>
<?php } ?>

Answer 1

Make a counter outside the function.

Inside the function, increment the counter. But before that, check if it countr % 3 == 0.

If so, show the big thumbnail.

<?php
 $counter = 0;
 foreach ($posts as $post) {  
        if($counter %3 == 0)
        {
           $big = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_600x200' );
        }else{
           $small = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail_200x100' );
        }

    if ( $big ) { ?>
        <img src="<?php echo $big['0']; ?>" />
    <?php }else{ ?>
        <img src="http://placehold.it/600x200/7f8c8d/ffffff" alt="Featured image missing"/>
    <?php } ?>
  counter++; //increase the counter
<?php } ?>

Answer 2

How about an indicator which increases in size for each post, starting with the value 3 and you always do a modulo

if(($i % 3) == 0) { 
  use big 
} else {
  use small
}
$i++;