Help retrieving related record

I have a database setup as follows:

CREATE TABLE `images`( 
   `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT, 
   `name` VARCHAR(20) NOT NULL, 
   `description` VARCHAR(255), 
   PRIMARY KEY (`id`)
 )  ENGINE=INNODB COMMENT='' ROW_FORMAT=DEFAULT;


CREATE TABLE `images_sizes` (
   `image_id` INT(11) UNSIGNED NOT NULL,
   `size` VARCHAR(7) NOT NULL,
   PRIMARY KEY  (`image_id`,`size`),
   CONSTRAINT `images_size_image_fk` FOREIGN KEY (`image_id`) REFERENCES `images` (`id`)
 )  ENGINE=INNODB COMMENT='' ROW_FORMAT=DEFAULT;

CREATE TABLE `advertisers`( 
   `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT, 
   `name` VARCHAR(50) NOT NULL,
   `website` VARCHAR(255),
   `contact_name` VARCHAR(50) NOT NULL,
   `contact_phone` VARCHAR(50) NOT NULL,
   `contact_email` VARCHAR(50) NOT NULL, 
   PRIMARY KEY (`id`)
 )  ENGINE=INNODB COMMENT='' ROW_FORMAT=DEFAULT;

CREATE TABLE `ad_blocks` (
  `id` int(11) unsigned NOT NULL auto_increment,
  `image_id` int(11) unsigned default NULL,
  `title` varchar(25) default NULL,
  `text` varchar(70) NOT NULL,
  `link_text` varchar(15) default NULL,
  `link_href` varchar(100) NOT NULL,
  `advertiser_id` int(11) unsigned NOT NULL,
  `is_active` tinyint(1) default '0',
  PRIMARY KEY  (`id`),
  KEY `image_id` (`image_id`),
  KEY `advertiser_id` (`advertiser_id`),
  CONSTRAINT `ad_block_advertiser_fk` FOREIGN KEY (`advertiser_id`) REFERENCES `advertisers` (`id`),
  CONSTRAINT `ad_block_image_fk` FOREIGN KEY (`image_id`) REFERENCES `images` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

CREATE TABLE `ad_block_activities` (
  `ad_block_id` int(11) unsigned NOT NULL,
  `user_name` varchar(32) NOT NULL,
  `action` varchar(32) NOT NULL,
  `activity_ts` int(11) NOT NULL,
  PRIMARY KEY  (`ad_block_id`,`activity_ts`),
  CONSTRAINT `activity_ad_block_fk` FOREIGN KEY (`ad_block_id`) REFERENCES `ad_blocks` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

When working with an AdBlock object how can I retrieve the associated Image? The fActiveRecord::build method is returning an error (as it should in this case). Do I need to change my design or is there a method I'm not seeing?