You could just run:
Fourier[ImageData[insert image here]]
and then run:
Abs[Fourier[ImageData[insert image here]]]
to get a rational version.
Finally you can run:
Image[Abs[Fourier[ImageData[insert image here]]]]
to get an image back.
Also on a side note in computer programs fourier transforms are not run in either the old method (slow fourier transforms?) or with fast fourier transforms but with a completely different algorithm.