Wordpress -- add description below to featured image - wordpress

Question

I would like to add an annotation below featured image box in post page ,saying
'Recommended image size: 1300px (width) x 340px (height)
Is it possible in wordpress 4.2.2.
The text is for the admin user for a better guidance.
Any help is highly appreciated. Thanks in advance.

Answer 1

Old post here, but I was trying to do this as well. A filter since WordPress 2.9.0 offers this capacity, like so:
/**
* Filters the admin post thumbnail HTML markup to return.
*
* #since 2.9.0
* #since 3.5.0 Added the `$post_id` parameter.
* #since 4.6.0 Added the `$thumbnail_id` parameter.
*
* #param string $content Admin post thumbnail HTML markup.
* #param int $post_id Post ID.
* #param int $thumbnail_id Thumbnail ID.
*/
return apply_filters( 'admin_post_thumbnail_html', $content, $post->ID, $thumbnail_id );
So use it like this:
function stackoverflow_30817906( $content, $post_id, $thumbnail_id ) {
if ( 'post' !== get_post_type( $post_id ) ) {
return $content;
}
$caption = '<p>' . esc_html__( 'Recommended image size: 1300px (width) x 340px (height)', 'i18n-tag' ) . '</p>';
return $content . $caption;
}
add_filter( 'admin_post_thumbnail_html', 'stackoverflow_30817906', 10, 3 );

Answer 2

If you want to do this without editing core files you could just use jQuery to add the text.
add_action('admin_footer', function() {
global $pagenow;
if('post.php' == $pagenow || 'post-new.php' == $pagenow) : ?>
<script>
jQuery(document).ready(function($) {
$('#postimagediv .inside').after('<div style="padding: 0 12px 12px;">Recommended image size: 1300px (width) x 340px (height)</div>');
});
</script>
<?php endif;
});
This solution isn't as 'clean' as making the change directly in the PHP file, but I think it is preferable to editing the core. With this solution you don't have to worry about remaking the change after a WordPress update.

Answer 3

You should go to wp-admin->includes->post.php
and around line 1295 (depending on version) you will find:
if ( !empty( $thumbnail_html ) ) {
$ajax_nonce = wp_create_nonce( 'set_post_thumbnail-' . $post->ID );
$content = sprintf( $set_thumbnail_link, $upload_iframe_src, $thumbnail_html );
$content .= '<p class="hide-if-no-js">' . esc_html__( 'Remove featured image' ) . '</p>';
}
change it to:
if ( !empty( $thumbnail_html ) ) {
$ajax_nonce = wp_create_nonce( 'set_post_thumbnail-' . $post->ID );
$content = sprintf( $set_thumbnail_link, $upload_iframe_src, $thumbnail_html );
$content .= '<p class="hide-if-no-js">' . esc_html__( 'Remove featured image' ) . '<span style="display:block;">Recommended image size: 1300px (width) x 340px (height)</span></p>';
}
It is not recommended to change core files as your changes can be lost during the update, but I don't recommend creating a plugin just for this small change.