Not exactly sure what that means, but you can do:
meanOfWholeImage = mean(grayImage(:));
zeroMeanImage = double(grayImage) - meanOfWholeImage;
If you want the mean within a window size, you can use imfilter
kernel = ones(9); % Whatever window size you want.
outputIMage = imfilter(grayImage, kernel);
If you want a uniform image, you can use
uniformImage = uint8(128*ones(rows, columns)); % Can use whatever numbers you want.
