"Note These APIs are meant primarily for ObjectDBX developers.
Preview thumbnail behavior reflect what the host application must
do to generate a preview.
For ObjectARX developers AutoCAD is taking care of all preview
thumbnail generation. ObjectARX users should leave these APIs
alone, and allow AutoCAD behaviors to prevail.
Most of the time, the AcDbDatabase::thumbnailBitmap() method
will return nothing, since the bitmap is not read into memory by
AcDbDatabase::readDwgFile(). To retreive the thumbnail from the
DWG file, (as opposed to directly from the db) the same existing
ObjectARX standalone functions are preferred. (for example