I could not resist and have just purchased Spine essentials

I have started to create my first image in sketchbook, a rather battered old voodoo doll.
The majority of the image will be static but I intend to animate the eyes so they go from black to glowing red and back to black.
I thought this would be a good project to start with as I only have to animate a small section of the image.
When I am more familiar with the app I could then look at adding subtle movements to the rest of the doll.
Now a quick question.
What file type should I save my image within sketchbook in order to export to Spine?