Duplicated and rotated the background layer 3 times then combined and cropped square so I had 4 fish swimming in a circle in a square image. Then applied Josh Summers Mathmap Droste9 code to make the spiral.
So no excuses now. Get GIMP get Mathmap and make your own Escher Droste!
More and better drostes here at
the Escher Droste Pool on Flickr