Currently there is a bug when using "Save for Web" in Photoshop (GIMP is also affected) while having the images dir set in Spine. To get around this you can just use "Save As" and make sure your image mode is set to 8bits/Channel.
If you're a GIMP user you can rollback to the previous version of Spine in the Settings menu. Also restarting Spine or changing the images dir to something else and then back again fixes it.
We'll have this bug fixed as soon as Nate comes back from his honeymoon on the 17'th