This isn't a bug. The background is marked as a background in the editor. This means it's not supposed to be exported.
There's a background
checkbox below the tree when you select an attachment/image.
This is currently a feature that allows you to put a background behind your Spine animation to see and animate it in context. It also prevents you from accidentally selecting the background by making it unselectable.
Marking it as a background is equivalent to telling Spine that the image is not supposed to be part of the skeleton.
EDIT:
@Nate:
Feature requests:
(1) Make the Background
state of attachments clearly visible in the tree, through a different icon or color or something.
(2) Give Slot
s a Background
checkbox so the slot and any of the attachments under it will not be included in the json.
Hmm. Maybe Background
should be labeled Guide
instead.