add_image_size not working with the_post_thumbnail

Question

Code in my funcitons.php

function custom_thumbs() {
    add_image_size( 'cover-size', 1600, 450, true ); 
}
add_action( 'after_setup_theme', 'custom_thumbs' );

Usage:

echo the_post_thumbnail($post->ID,'cover-size');

but this is the result:

<img width="3648" height="2736" src="http://localhost/sonru/wp-content/uploads/2015/12/DSC04656.jpg" class="attachment-9 size-9 wp-post-image" alt="DSC04656" cover-size="" srcset="http://localhost/sonru/wp-content/uploads/2015/12/DSC04656-300x225.jpg 300w, http://localhost/sonru/wp-content/uploads/2015/12/DSC04656-600x450.jpg 600w" sizes="(max-width: 3648px) 100vw, 3648px">

What Am I missing here?

-Edit-

I'm already regenerating thumbnails using the regenerate thunbnails plugin

Answer 1

You may need to regenerate your images. There are plugins to accomplish this or re-upload your image and see if the size doesn't take affect. It won't happen automatically to existing files.

First, check your uploads directory to see if the files exists that way you know they are being generated. Then try another function to output the results.

What do you end up with when you use:

$attachment_id = get_post_thumbnail_id( $post->ID ); 
$img_src = wp_get_attachment_image_url( $attachment_id, 'cover-size' ); 
$img_srcset = wp_get_attachment_image_srcset( $attachment_id, 'cover-size' );

Also if may be handy to output your meta data for that attachment - specifically to see the size information:

print_r ( wp_get_attachment_metadata( $attachment_id) );

The above wp_get_attachment_image_srcset requires WordPress 4.4 and you can find more information about Responsive Images in WordPress 4.4.

Assuming the files exist:

• the_post_thumbnail ( $size, $attr )

• get_the_post_thumbnail ( int|WP_Post $post = null, string|array $size = 'post-thumbnail', string|array $attr = '' )

Are you in a Loop or outside of it?

If you're inside try the_post_thumbnail ('cover-size');

otherwise echo get_the_post_thumbnail ($post,'cover-size');

Answer 2

You are using the wrong function:

• the_post_thumbnail() does not accept the post ID as value, but the image size.

• get_the_post_thumbnail() is what you need and maybe trying to use. The first parameter accept the post ID and the second parameter accepts the image size to use as in your question

SOLUTION

Swap the_post_thumbnail() with get_the_post_thumbnail()

EDIT

Just a note, make sure that you add your new image size after you have added support for thumbnails. Also, you would want to add a new post thumbnail size with set_post_thumbnail_size()