I am currently doing this here by simply letting mysql compare the first 256 bytes of each image. While images could be created that would cause a false positive with this test it is not a problem in this application. I am simply trying to keep the exact same file from being uploaded more than once.

CREATE TABLE `image` (
  `id` mediumint(9) NOT NULL auto_increment,
  `type` varchar(20) default 'jpeg',
  `mime_type` varchar(20) default 'image/jpeg',
  `image` mediumblob,
  `size` mediumint(9) default NULL,
  `width` int(11) default NULL,
  `height` int(11) default NULL,
  `user_id` mediumint(9) default NULL,
  `created` datetime NOT NULL default '0000-00-00 00:00:00',
  `copyright_id` mediumint(9) default NULL,
  `comments` text,
  `hidden` enum('true','false') default 'false',
  PRIMARY KEY  (`id`),
  UNIQUE KEY `uk_image_image` (`image`(256))
)
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