When your website means business.

Custom Admin, Users, WP/CP | Type: PHPAdd Message on Thumbnail Metabox

Sometimes you need to change the message.

/** Add Featured Image Text above thumbnail **/
function filter_featured_image_admin_text( $content, $post_id, $thumbnail_id ){
    $help_text = '<p>' . __( '<i>Please use an image that is 800 pixels wide (min).</i>', 'ClassicPress' ) . '</p>';
    return $help_text . $content;
}
add_filter( 'admin_post_thumbnail_html', 'filter_featured_image_admin_text', 10, 3 );

Reference Links