functions.php

<?php

/**
 * Post Image with Fallback
 *
 */
function be_post_image_with_fallback() {
  if ( ! is_singular() && genesis_get_option( 'content_archive_thumbnail' ) ) {
		$img = genesis_get_image( array(
			'format'  => 'html',
			'size'    => genesis_get_option( 'image_size' ),
			'context' => 'archive',
			'attr'    => genesis_parse_attr( 'entry-image' ),
		) );
    
		if( empty( $img ) )
			$img = be_no_thumbnail( genesis_get_option( 'image_size' ) );

		if ( ! empty( $img ) )
			printf( '<a href="%s" title="%s">%s</a>', get_permalink(), the_title_attribute( 'echo=0' ), $img );
	}

}
add_action( 'genesis_post_content', 'be_post_image_with_fallback', 9 );
remove_action( 'genesis_post_content', 'genesis_do_post_image' );

/**
 * Default Thumbnail
 * @author Bill Erickson
 * @link http://www.billerickson.net/default-image-for-post-thumbnails/
 *
 * @param string image size
 */
function be_no_thumbnail( $size ) {
  return '<img width="'.get_option($size.'_size_w').'" height="'.get_option($size.'_size_h').'" src="'.get_bloginfo('stylesheet_directory').'/images/no-thumbnail.gif" />';
}