This is my answer to your last question. If X is a grayscale RGB image, this will count the white pixels:
idy = all(X,3); % R==G==B
out = sum(idy(:)); % count 1's
Note that you should not store your image as a .jpg file, as this is a lossy format. .png would be more suitable.
Anyway, here is one easy way to count the pixels with values less than 127 ("black"):
>> X = imread('1_2.jpg');
>> Y = X>127;
>> L = sum(Y(:)) % Light
L = 97146
>> D = sum(~Y(:)) % Dark
D = 21574
And the total pixels in both of these is equal to the total pixels in the image:
>> L+D
ans = 118720
>> prod(size(X))
ans = 118720
