If you want to use a specific image for the ground (with different lighting and capture parameters or even with a human figure) you can import it into Nebulb as a "Master Image" and then select it as the image for the ground.
And in case the master image includes some element that was not in the rest of the images used for stacking (such as, for example, a human figure protruding above the horizon) what you can do is:
- Select the master image in the image selection drop-down list (on the left of the bottom toolbar).
- Edit the sky mask, excluding non-sky objects in that image (such as, for example, a person posing).
- Refresh the sky mask.