wp_custom_featured_image.php

/*
 * Customize the Featured Image metabox w/ info-text
 * 
 */
function mim_remove_featuredimage_metabox() {
    remove_meta_box( 'postimagediv', 'inspire', 'side' );
}
add_action( 'admin_head', 'mim_remove_featuredimage_metabox' );

function mim_custom_featuredimage_metabox( $post ) {
	
	$thumbnail_id = get_post_meta( $post->ID, '_thumbnail_id', true );
	
	echo "Upload an image that is at least 365px-wide by 120px-tall. Larger images will be auto-cropped.";
	echo _wp_post_thumbnail_html( $thumbnail_id, $post->ID );
	
}
function mim_add_featuredimage_metabox() {
	add_meta_box( 'postimagediv', __( 'Featured Image' ), 'mim_custom_featuredimage_metabox', 'inspire', 'side', 'default' );
}
add_action( 'admin_head', 'mim_add_featuredimage_metabox' );