DCT and mean difference of an image If we subtract the mean of image from the DCT coefficients of an image 


what does it signifies?
and what type of distribution does it follow?


Example in MATLAB:

i=imread('cameraman.tif');
c=mean2(i);
d=dct2(i);
f=c-d;


what does f signifies?
You can investigate that if the size of the DCT block (which is the whole image in here) is $x\times y$, then 

c=d(1,1)/sqrt(x*y);


That is, the DC element of the DCT matrix (i.e. d(1,1)) is actually the mean of the image scaled by $\sqrt{xy}$.

So you are basically subtracting a scaled version of the DC component form all coefficients, which makes not much sense at the moment, except if you provide more context.