The specific problem you're having is that you created a bunch of variables called A1, A2, etc., but in the loop you're trying to reference A(2), which is not the same thing as A2.
You could keep everything in a single array A. How you do that depends on the files. If they are all B&W images of the same size, you could keep them in a 3-D double array:
A(:,:,i) = double(imread...
Then, at the end,
average_image = mean(A,3);
Given that you're trying to do averaging, I assume your images are at least the same size. If they're color images, things get a bit gnarlier, but you could still do it with 4-D arrays (depending on what you mean by averaging the images).
EDIT TO ADD: OK, given your comment, if you're dealing with true-color images, imread will return them as 3-D arrays (m-by-n-by-3), so you could store the b different images in a 4-D array (m-by-n-by-3-by-b):
for i = 1:b
A(:,:,:,i) = double(imread(...
If the "average" image is just the mean of each of the 3 color planes individually, then you can do
average_image = mean(A,4);
If you want something more complicated than that, then... well, I'm sure it's possible, but I don't know what it would look like.