In the normal B&N firmware, no. The reason it defaults back to the authors is that those files are stored on an internal disk partition that is not shared when you plug the device in to your PC, so the NST can still access them. Your personal screensaver images are on a partition that is shared, so becomes unavailable to the NST so long as you have it plugged in and mounted on your PC.
If you root the device, you can try making changes, but I haven't had much luck. The Bronte sisters is the first image in that directory, so I tried adding an image with a name that appeared first, but it still showed the girls on initial boot. I haven't tried deleting them yet, or replacing that image.
I'm not aware of any way to change this. Considering that it's just when the NST is plugged in, it hasn't really bothered me. I tend not to be focused on the screen saver at those times, but rather on whatever other task is at hand. Plus, it always reverts back to my designated screensaver choice once it's unplugged.