When trying to attach an ECW raster image to a DWG file the following information message is being displayed.
Another message is displaying at the bottom in the Message Center, "Raster can not be read".
While working in DWG Workmode you can attach an ECW raster image by adding a DWG Capability into you configuration files.
If you add the following configuration variable to your User Configuration File (UCF) or Project Configuration File (PCF) it will allow this capability but only during the current session of MicroStation.
By default the UCF and PCF file can be located in the following folder:
C:\ProgramData\Bentley\MicroStation V8i (SELECTseries)\WorkSpace\
_USTN_CAPABILITY_DWG < +CAPABILITY_RASTER_ALL_TYPES
This will also allow you to attach of all supported raster file formats.
Please note that this cannot be saved to DWG using versions of MicroStation V8i (08.11.09.459) or older.
In order to retain the ECW raster attachment when saving to DWG you need to upgrade to MicroStation version 08.11.09.676 which has introduced this capability. As above, place the following configuration variable in your UCF or PCF to include this capability.
now you need to save from DGN to DWG format but without attaching the ECW raster yet
when saving to DWG press option button and in Save as DWG\DXF options: select DWG version to 2013/2014
Now in DWG format with version 2013 try to attach ECW raster again and you will see the attachment is possible also when you save and close file the ECW attachment is retained.
http://communities.bentley.com/products/interoperability/f/157/p/8696/14356#14356