With the time images used as Dashboard background can become obsolete and we may need to delete some of them.
In the old Dashboard these images were stored in the '\nimsoft\probes\service\wasp\umpmedia' directory and in the 'dbo.tbnMedia' table in the database and the solution was to remove them from both spots.
In the recent versions of the Dashboard (hmtl5) any image that is added is only stored in the 'dbo.umpMedia' table. Removing obsolete images is as simple as executing a query like the following:
delete from umpMedia where name like 'file_name'