You cannot use an existing *.img file in BC to load parts of it elsewhere. It's possible to split an image using software to make a simple preview map to install on PC which you can work with in MapSource or BaseCamp but search won't work and any so re-compiled img map would be useless on a device for that reason.
BaseCamp itself doesn't 'save' maps to devices or cards/flash drives, it invokes MapInstall to do that job from a PC Install version [gmap folder usually]. You can simply open MapInstall directly and use it to compile the map parts you want into an img file. If MI is patched you can send the .img to any drive, even your PC's system drive. Read up: [Only registered and activated users can see links. ]. Start with this one: [Only registered and activated users can see links. ] and look for more under 'Mapsource and other programs' etc.