When registering your raster image, you need to know the map coordinates that correspond to your image. If your raster map image shows a graticule (a grid of longitude/latitude lines), you can determine map coordinates by noting the longitude/latitude labels along the graticule.
If your map does not show a graticule, you may be able to determine map coordinates by locating prominent map features, for example, the Northwest corner of a region, and then using another MapInfo Pro table as a reference to determine the coordinates of those prominent features.
For a discussion of this process, see Choosing Control Points from an Existing Map.