functions.php

<?php

/**
 * Black and White Image
 * @url http://ottopress.com/2011/customizing-wordpress-images/
 *
 * @param array $meta
 * @return array $meta
 */
function be_bw_filter($meta) {
	$file = wp_upload_dir();
	if( isset( $meta['sizes']['be-subpage-bw']['file'] ) )
		$file = trailingslashit($file['path']).$meta['sizes']['be-subpage-bw']['file'];
	else
		$file = trailingslashit( $file['basedir'] ) . $meta['file'];
	list($orig_w, $orig_h, $orig_type) = @getimagesize($file);
	$image = wp_load_image($file);
	imagefilter($image, IMG_FILTER_GRAYSCALE);
	switch ($orig_type) {
		case IMAGETYPE_GIF:
			imagegif( $image, $file );
			break;
		case IMAGETYPE_PNG:
			imagepng( $image, $file );
			break;
		case IMAGETYPE_JPEG:
			imagejpeg( $image, $file );
			break;
	}
	return $meta;
}
add_filter('wp_generate_attachment_metadata','be_bw_filter');