Freeing Unused Space SQL Server Table
The only thing we have done is replaced ScanImage on every row with a
  much smaller image (this is how so much unused space is there)


From doing some experimentation the most space effective method would be to drop the allocation unit and repopulate it (if you have a maintenance window to do this in).

Example code that achieved the best space reduction for me with the table structure in the question is:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

SET XACT_ABORT ON;

BEGIN TRAN

SELECT [ImageID],
       [ScanImage]
INTO   #Temp
FROM   [dbo].[MyTableName]

ALTER TABLE [dbo].[MyTableName]
  DROP COLUMN [ScanImage]

/*Allocation unit not removed until after this*/
ALTER INDEX PK_Image ON MyTableName REBUILD

ALTER TABLE [dbo].[MyTableName]
  ADD [ScanImage] IMAGE NULL

UPDATE [dbo].[MyTableName]
SET    [ScanImage] = T.[ScanImage]
FROM   [dbo].[MyTableName] M
       JOIN #Temp T
         ON M.ImageID = T.[ImageID]

DROP TABLE #Temp

COMMIT 


Everything is in a transaction so if the machine crashes it will be rolled back. Could probably do with some error handling or at least SET XACT_ABORT ON. I used SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; to prevent any concurrent modifications from happening during or after the copy and being lost.

The number of LOB pages reserved after reducing the size of an image in all rows was as follows:


+--------------------------------------------------+---------------------+-------------------------+
|                      Event                       | lob_used_page_count | lob_reserved_page_count |
+--------------------------------------------------+---------------------+-------------------------+
| Inserted 10,000 rows with 100,000 byte data each |              135005 |                  135017 |
| Updated all rows to 10,000 byte image data       |               31251 |                  135012 |
| Reorganize                                       |               23687 |                   25629 |
| Drop and re-add image data                       |               13485 |                   13489 |
+--------------------------------------------------+---------------------+-------------------------+