I am not sure if this is a bug or something I am doing wrong, but this is what happened.
I created a skeleton and animation, using a template image as my guide as suggested in the spine tutorial page. It was attached to the root, and when I exported the project, the resulting packed image included that template image. I didn't want that, so I deleted the slot with the template image from my project, saved, and exported once more. The new resulting packed image is basically the same as the first, but with just empty space where the template image used to be.
I've tried again, but the empty space still ends up in the packed image.