The GeoTIFF import is really designed to import DEMs saved as GeoTIFFs, and not images.
If you want to display the image in RockPlot2D, you can import it through the File | Import | Raster Image menu. You can use the world file to georeference the image, as long as the coordinate system is the same (the program does not read the PRJ file).
If you want to drape the image over the RwGrd file, the Image | Drape program allows you to select the image and world file, along with the RwGrd file.