The intro of the package:
This package is mainly not intended to be used directly. Instead, developers of packages that implement FFTs (such as FFTW.jl or FastTransforms.jl) extend the types/functions defined in AbstractFFTs. This allows multiple FFT packages to co-exist with the same underlying fft(x) and plan_fft(x) interface.
Like @rafael.guerra said, you could use FFTW.
using FFTW
img=[sin(2*pi*x*10/256) for x in 1:256, y in 1:256]
fft(img)