mysql> desc Gallery;

+--------------+------------------+------+-----+-------------------+-----------------------------+
| Field        | Type             | Null | Key | Default           | Extra                       |
+--------------+------------------+------+-----+-------------------+-----------------------------+
| id           | int(10) unsigned | NO   | PRI | NULL              | auto_increment              |
| title        | varchar(255)     | NO   |     | NULL              |                             |
| subtitle     | varchar(255)     | NO   |     | NULL              |                             |
| description  | varchar(5000)    | NO   |     | NULL              |                             |
| date         | datetime         | NO   |     | NULL              |                             |
| isActive     | tinyint(1)       | NO   |     | NULL              |                             |
| lastModified | timestamp        | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+--------------+------------------+------+-----+-------------------+-----------------------------+

mysql> desc Image;
+--------------+------------------+------+-----+-------------------+-----------------------------+
| Field        | Type             | Null | Key | Default           | Extra                       |
+--------------+------------------+------+-----+-------------------+-----------------------------+
| id           | int(10) unsigned | NO   | PRI | NULL              | auto_increment              |
| galleryId    | int(10) unsigned | NO   | MUL | NULL              |                             |
| description  | varchar(250)     | YES  |     | NULL              |                             |
| path         | varchar(250)     | NO   |     | NULL              |                             |
| lastModified | timestamp        | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+--------------+------------------+------+-----+-------------------+-----------------------------+

UPDATE table set order=order+1 where order>='orderValueOfItemYouCareAbout';

mysql> SELECT id, galleryId, description, displayOrder FROM Image ORDER BY displayOrder;
+-----+-----------+----------------------------------+--------------+
| id  | galleryId | description                      | displayOrder |
+-----+-----------+----------------------------------+--------------+
| 271 |        20 | NULL                             |            1 |
| 270 |        20 | Tracks leading into the ocean... |            2 |
| 278 |        20 | NULL                             |            3 |
+-----+-----------+----------------------------------+--------------+
3 rows in set (0.00 sec)

UPDATE Image SET displayOrder =
  CASE displayOrder
    WHEN 2 THEN 3
    WHEN 3 THEN 2
  END
WHERE galleryId = 20
AND displayOrder BETWEEN 2 AND 3;

mysql> SELECT id, galleryId, description, displayOrder FROM Image ORDER BY displayOrder;
+-----+-----------+----------------------------------+--------------+
| id  | galleryId | description                      | displayOrder |
+-----+-----------+----------------------------------+--------------+
| 271 |        20 | NULL                             |            1 |
| 278 |        20 | NULL                             |            2 |
| 270 |        20 | Tracks leading into the ocean... |            3 |
+-----+-----------+----------------------------------+--------------+
3 rows in set (0.00 sec)