After you select an image from the local gallery (using the button "Select Image") you need to press "Apply FFT" in order to actually transform the image into a  matrix of grayscale values. After that you need to slide to the left and you will get: "Amplitude FFT Image", "Phase FFT Image", "FFT Fourier log image", and also "Original Image after applying the inverse FFT", where FFT = Fast Fourier Transform. The image which you bring to the app should be a square image of 2^n pixels (e.g.: 1024 x 1024)