An external image source refers to devices that Neurolucida is not configured to control (e.g., laser confocal microscope).
Use to inform Neurolucida that you will be loading images from an external image source while controlling the stage via Neurolucida.
The command allows you to enter image dimensions that will override the dimensions/settings associated with your system's configuration. This will allow Neurolucida to calculate the external image coordinates and scaling correctly.