Change Featured Image / Thumbnail CMS Description

The following will add help text to the initial “Set featured image” text.

Add the following to your theme’s functions.php. Replace “Your custom text goes here” with your help text.

Tested and works.

function custom_featured_image_text( $content ) {
    return '<p>' . __('Your custom text goes here') . '</p>' . $content;
}
add_filter( 'admin_post_thumbnail_html', 'custom_featured_image_text' );

The following will add help text to the “Click the image to edit or update” text after you upload an image.

function custom_featured_image_text_2( $content ) {
    return str_replace(__('Click the image to edit or update'), __('Your custom text goes here'), $content);
}
add_filter( 'admin_post_thumbnail_html', 'custom_featured_image_text_2' );