At any time you can remove the master image you have imported for the ground by simply pressing the following button and confirming the confirmation message that will appear:

If you remove the master image from Nebulb (or have not imported it) the section with information about the master image will be empty and you will not be able to select it from the drop-down list in the lower toolbar either.

However, you can re-import the master image at any time. In fact, the normal mechanism for changing the master image (in case you want to try several) is to first remove the one you have previously imported and then import the new master image.

IMPORTANT: Removing the master image does NOT delete the image file, it simply removes it from Nebulb (keeping the image file intact).