You certainly don't need Raster Design to just import them. Instead of doing it via the typical image attach you have to insert them through the Map menu. Map > Image > Insert and then you'll see that it gives options on placement and such. If your world file is in the same directory with the image it should already note that and place it properly based on your coordinate system settings. The only thing you will likely need to do is install the Raster Design object enablers for Civil 3d. This allows you to attach the other formats such as jpeg2000, mrsid, etc.