In case you have doubts about the images you have imported into Nebulb or, for whatever reason, you have imported an image that you do not want to be stacked, you can discard all the imported images so that you can perform the import again.
And to delete from Nebulb all the images you have imported you just have to press the following button:
When you do so, you will get a confirmation message and, if you confirm it, all imported images will be discarded from Nebulb.
IMPORTANT 1: Deleting the imported images in Nebulb does NOT delete the image files, it simply removes them from Nebulb (keeping the image files intact).
IMPORTANT 2: In case you had already defined the mask for the sky, it will not be deleted even if you delete the imported images, so that when you import the images again, the mask you had previously created will still be there.