In Didger 3, the File | Create Surfer Base Map command only supports bitmaps that are imported into a vector project. If you have a bitmap imported into a raster project, it will not be exported into Surfer. If this is the case, export the bitmap as a GeoTIFF and import it into a vector project and use the Create Surfer Base Map command.
Didger 4 and Didger 5 solve this issue as there is only one project type.
Updated March 20, 2014