The small print: "File comment: Image file to be cleaned up by mask file, and alpha added.. eventually automated."
Easy enough one at a time....eventually automated ...is less easy.
It should be possible to write a Gimp script that will batch process pairs of images but a better tool for this is commandline Imagemagick (IM).
Still needs wrapping in a batch / bash file with file names replaced by variables, the basic command is
magick image-0007.png image-0008.png -alpha Off -compose CopyOpacity -composite masked.png
gives this:
Attachment:
masked.jpg [ 124.22 KiB | Viewed 3017 times ]
see:
http://www.imagemagick.org/Usage/masking/#masks note: IM6 uses convert IM7 uses magick