Here are the images I tried with your code, i see a change in both images. There is more distortion for image with noise magnitude of 100. Can you attach your images? What did you mean by accuray? Is it the same as amount of distortion or something else?
% Original Image
figure
A=imread('coins.png');
K = imcrop(A,[116 50 675 534]);
I=uint8(double(K));
imshow(I)
% Image with Noise magnitude = 20
figure;
A=imread('coins.png');
K = imcrop(A,[116 50 675 534]);
b = 20*randn(size(K));
I=uint8(double(K)+b);
imshow(I)
% Image with Noise magnitude = 100
figure;
A=imread('coins.png');
K = imcrop(A,[116 50 675 534]);
b = 100*randn(size(K));
I=uint8(double(K)+b);
imshow(I)
