Dependence of $\tt n$ in $\tt fft(data,n)$ in auto correlation of data The followings are results of autocorrelation of an 'image'(1*) (subplot 221) using xcorr, n point fft, and 2n-1 point fft. I observe that I get different results. Why does it happen?

PS: result at subplot 223 is the expected image.

(1*) In fact it is identified as 'integrated image' in the paper doi:10.1038/nature11578 by Dr. Jacopo Bertolotti, which can not be considered as a general image that has both magnitude and phase information.
Doing an $N \times M$ point FFT for that sized image to calculate the autocorrelation will result in time-aliasing. 

To multiply two signals of length $N$ in the frequency domain, the FFT length must be $2N-1$.