Mapping the Value of a Sample in a 2D DFT to Cycles/Pixel
Well the Nyquist frequency is 0.5 cycles / pixel and the range of frequencies can be considered to range from -0.5 to +0.5 cycles / pixel. So if the dimension in a given axis is N pixels then each pixel is 1 / N cycles per pixel.

Depending on your particular DFT/FFT implementation you may have zero frequency in the middle of the image (M/2, N/2) or in the corner (0, 0). For the former case the spatial frequency at u, v will be:

(M/2 - u) / M, (N/2 - v) / N