post thumbnail

Here’s how to define a default post thumbnail that works with Genesis’s genesis_get_image() function. Be sure to place a default image called thumb.jpg in your child theme’s images folder.

/** Define a default post thumbnail */
add_filter('genesis_get_image', 'default_image_fallback', 10, 2);
function default_image_fallback($output, $args) {
	global $post;
	if( $output || $args['size'] == 'full' )
		return $output;

	$thumbnail = CHILD_URL.'/images/thumb.jpg';

	switch($args['format']) {

		case 'html' :
			return '<img src="'.$thumbnail.'" class="attachment-'. $args['size'] .'" alt="'. get_the_title($post->ID) .'" />';
			break;
		case 'url' :
			return $thumbnail;
			break;
	   default :
		   return $output;
			break;
	}
}

Updated from Lori Berkowitz

See also: Nick the Geek’s tutorial – Genesis Explained Image Functions