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 sides 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 completed) are two things: