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 “Master image” and then select it as the image for the ground.
In the following screenshot you can see it:
And in case the master image includes some element that was not in the rest of the images used for stacking (like, for example, a human figure protruding above the horizon) you can do the following:
1. Select the master image in the image selection drop-down below.
2. Edit the mask for the sky, excluding non-sky objects in that image (such as a person posing).
3. Refresh the sky mask.
Once this is done, select the “Aligned stars (sky) + Master image (ground)” image again and you will see the sky resulting from the stacking together with the ground of the master image.