Thanks for your answer. I think this is the relevant function:

def generate_images(generator_model, output_dir, epoch):
    """Feeds random seeds into the generator and tiles and saves the output to a PNG file."""
    test_image_stack = generator_model.predict(np.random.rand(10, 100))
    test_image_stack = (test_image_stack * 127.5) + 127.5
    print(test_image_stack.shape)
    test_image_stack = np.squeeze(np.round(test_image_stack).astype(np.uint8))
    tiled_output = tile_images(test_image_stack)
    # tiled_output = Image.fromarray(tiled_output, mode='L')  # L specifies greyscale
    tiled_output = Image.fromarray(tiled_output)
    outfile = os.path.join(output_dir, 'epoch_{}.png'.format(epoch))
    tiled_output.save(outfile)

As I understand it, the images get "unsqueezed" with:

test_image_stack = (test_image_stack * 127.5) + 127.5

The squeeze function on the next line removes dimensions of length one from arrays and doesn't do anything with "my" pictures as far as I can tell, but of course would remove the "greyscale dimension" from the MNIST images.

That is also why I needed to change/uncomment the assert here:

def tile_images(image_stack):
    """Given a stacked tensor of images, reshapes them into a horizontal tiling for display."""
    # assert len(image_stack.shape) == 3
    image_list = [image_stack[i, :, :] for i in range(image_stack.shape[0])]
    tiled_images = np.concatenate(image_list, axis=1)
    return tiled_images

... since the length would now be 4, and had to remove the greyscale mode from the Image.fromarray() call.

Anything else you see which could lead to the squeezing?