Notes for Generating MicroStation Thumbnails
To extract a thumbnail image from a file, the file must first contain a thumbnail image. When a DGN file is closed, MicroStation takes a screen capture of the lowest numbered view that is open in the DGN. This captured image becomes the thumbnail for the DGN file.
Another way you can get thumbnails is to create a saved view named "preview" in each DGN file. If this saved view exists, MicroStation will use the contents of it as the thumbnail rather than the lowest numbered open view. Or, if the MicroStation configuration variable MS_THUMBNAIL is defined, MicroStation takes whatever the variable is set to and looks for a saved view of the same name to generate the thumbnail.