I made some progress on this. Now I use the Grayscale command (num_output_channels = 1) to get a gray picture. However, she still has 3 channels. I have to use resize_ to get what I want [139,1,493,733] instead of [139,3,493,733]. If I have 3 channels, then matplotlib displays the image as gray. If 1 channel is greenish. This is fine?