how we use frequency domain analysis for image processing.

i want to do my thesis on image processing in frequency domain. i have done some work in spatial domain.but i have no idea about image processing in frequency domain. first i want to start image processing by using Fourier transform . i use the code a=imread('cameraman.tif'); imshow(a) b=fft(a); imshow(b) but i can't understand how the output is useful.it is a general some image .please tell me how i use the output of fft.