In phpMyAdmin, enter this at SQL:
DROP TABLE `mybb_icons`
Then:
CREATE TABLE IF NOT EXISTS `mybb_icons` (
`iid` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(120) NOT NULL DEFAULT '',
`path` varchar(220) NOT NULL DEFAULT '',
PRIMARY KEY (`iid`)
);
INSERT INTO `mybb_icons` (`iid`, `name`, `path`) VALUES
(1, 'Bug', 'images/icons/bug.gif'),
(2, 'Exclamation', 'images/icons/exclamation.gif'),
(3, 'Question', 'images/icons/question.gif'),
(4, 'Smile', 'images/icons/smile.gif'),
(5, 'Sad', 'images/icons/sad.gif'),
(6, 'Wink', 'images/icons/wink.gif'),
(7, 'Big Grin', 'images/icons/biggrin.gif'),
(8, 'Tongue', 'images/icons/tongue.gif'),
(9, 'Brick', 'images/icons/brick.gif'),
(10, 'Heart', 'images/icons/heart.gif'),
(11, 'Information', 'images/icons/information.gif'),
(12, 'Lightbulb', 'images/icons/lightbulb.gif'),
(13, 'Music', 'images/icons/music.gif'),
(14, 'Photo', 'images/icons/photo.gif'),
(15, 'Rainbow', 'images/icons/rainbow.gif'),
(16, 'Shocked', 'images/icons/shocked.gif'),
(17, 'Star', 'images/icons/star.gif'),
(18, 'Thumbs Down', 'images/icons/thumbsdown.gif'),
(19, 'Thumbs Up', 'images/icons/thumbsup.gif'),
(20, 'Video', 'images/icons/video.gif');
Go to to the _datacache table, and empty the contents of the posticons "cache" field.