The image that Nebulb selects as a base when loading the images is always the one that is temporarily in the center (it cannot be changed). This selection is made by Nebulb to ensure that during the stacking process the displacement of the stars to each side is symmetrical, which allows to obtain better results in terms of noise reduction (especially on the edges of the images).
Therefore, before stacking, it is not possible to change the base image.
That said, what can be done (after the stacking process is finished) are two things (the first one to leave the sky at the desired time of the night and the second one for the ground part):
1. select the sky for the desired time of night
Sky according to the base image automatically selected by Nebulb
Modification of the time of the night for which you want to have the sky: first move the time slider (1) and then press the button (2)
Image with the sky according to the new selected base image
This is especially useful for meteor shower photography, where you want the radiant to be in the position where it looks best and that often does not coincide with the central moment of the night.
2. Select a master image for the ground
In case you want to use a specific image for the ground (with different lighting and capture parameters), you can always import it into Nebulb as “Master image” and then select it as the image for the ground.