[dcanelhas]

Indeed, the frequencies get higher further away from the center of the FFT image with the center being the "DC" component of the signal. The brightness, as you note, correspond to the amplitude of a given frequency.

you can think of it as a polar plot where: radius = frequency, angle = angle (as in points along the horizontal direction in the FFT correspond to e.g. vertical stripes in the image), brightness = power

There is an imaginary part to the FFT that contains the phase-shifts needed to add everything back up into the original image. You may have noticed that the "FFT of the image" is symmetric, so something else should be needed to compute the inverse.