How to find phase values on precise pixel locations of a given Image?
You can't find the precise phase value of a given pixel because of the uncertainty principle. You can only find the phase value for the pixel's neighborhood using short-time Fourier tranform. 

If you want to know the phase for pixel (100,100), ou have to take a neighborhood, for instance (50:150,50:150) and calculate the 2D Fourier transform on this window.