I want to average some .jpg images which are corrupted by zero-mean Gaussian additive noise. After searching around, I figured out to add the image matrices and divide the sum by the number of matrices. However, the resultant image is totally black. Normally when the number of image increases then the resultant image gets better. But when I use more images it gets darker.

I am using 800x600 black and white .jpg images. Here is the script I used:

image1 = imread ('PIC1.jpg');
image2 = imread ('PIC2.jpg');
image3 = imread ('PIC3.jpg');
image4 = imread ('PIC4.jpg');

sum = image1 + image2 + image3 + image4; 
av = sum / 4; 
imshow(av);