Suggested fix for http://wordpress.org/plugins/external-featured-image/

nelio-efi-main.php

<?php

// Overriding post thumbnail when necessary
add_filter( 'post_thumbnail_html', 'nelioefi_replace_thumbnail', 10, 5 );
function nelioefi_replace_thumbnail( $html, $post_id, $post_image_id, $size, $attr ) {
	$image_url = get_post_meta( $post_id, '_nelioefi_url', true );

	if ( $image_url && strlen( $image_url ) > 0 ) {
		$width = false;
		$height = false;
		$additional_classes = '';

		global $_wp_additional_image_sizes;
		if ( is_array( $size ) ) {
			$width = $size[0];
			$height = $size[1];
		}
		else if ( isset( $_wp_additional_image_sizes[ $size ] ) ) {
			$width = $_wp_additional_image_sizes[ $size ]['width'];
			$height = $_wp_additional_image_sizes[ $size ]['height'];
			$additional_classes = 'attachment-' . $size . ' ';
		}

		if ( $width && $width > 0 ) $width = "${width}";
		else $width = '';

		if ( $height && $height > 0 ) $height = "${height}";
		else $height = '';

		if ( isset( $attr['class'] ) )
			$additional_classes .= $attr['class'];

		$html = sprintf(
			'<img src="%s" width="'.$width.'" height="'.$height.'" class="wp-post-image nelioefi" />',
			$image_url, $width, $height, $additional_classes );
	}

	return $html;
}


add_action( 'the_post', 'nelioefi_fake_featured_image_if_necessary' );
function nelioefi_fake_featured_image_if_necessary( $post ) {
	if ( is_array( $post ) ) $post_ID = $post['ID'];
	else $post_ID = $post->ID;

	$has_nelioefi = strlen( get_post_meta( $post_ID, '_nelioefi_url', true ) ) > 0;
	$wordpress_featured_image = get_post_meta( $post_ID, '_thumbnail_id', true );

	if ( $has_nelioefi && !$wordpress_featured_image )
		update_post_meta( $post_ID, '_thumbnail_id', -1 );
	if ( !$has_nelioefi && $wordpress_featured_image == -1 )
		delete_post_meta( $post_ID, '_thumbnail_id' );

}