Python-Fu/A Path To Circle Map...
1. Open an image with a face, draw a vector (closed loop vector) around the face (without the ears, just the face).
2. Run plug-in, choose number of sample points (100 is default) or could be thought of as sectors around the circle.
3. Wait, it'll create circle face image on a new image.
4. That's it enjoy.
Note: If you want to map a whole rectangle of source image to circle, make sure you select all turn to path, then extend canvas size and layer to image size such that the vector/path is WITHIN the canvas and not at the boundary because sometimes it errors out because of bounds.
You can see more examples on my CircleArt5 twitter account: https://twitter.com/CircleArt5
Here's a sample run (at 100 sample points, you can see how the circle is divided into circular pixels sort'a).
Here's a sample run (at 400 sample points, you see less circular pixels, looks much better).