All the templates have a GIMP format, but the cover art template. That's only offered as .png. Is there a reason for this? The .png even says on it something like - remember to delete this layer. But with a png file there's no layer to delete.
It would be easier for me if this template was offered as a GIMP file, just like the other templates.
I would also find a GIMP template to be easier. However, for now you can:
Granted, I'm using version 2.63, but I think this should still apply.