ifft2 works correctly.
You can demonstrate this with the following code
img = rgb2gray(imread('peppers.png')); % example image
img = img(1:256, 1:128); % make it 256x128
img = double(img)/256; % double, scale to 0-1
imshow(img); % have a look
imgTrans = fft2(img); % transform
imgRestored = ifft2(imgTrans); % inverse transform
figure; imshow(imgRestored); % looks ok
There must be some other problem with the way you generate the 256x128 matrix.
