title: numpy.ones results in red, green, blue, and black bars on image
body: <p>I have the following code:</p>
  
  <pre><code>from PIL import Image
  import numpy as np
  
  a = np.ones((512, 256, 3)).astype(int)*255
  image = Image.fromarray(a, "RGB")
  image.save("test.png", "PNG")
  </code></pre>
  
  <p>I would expect this to result in a white image of size 512x256.
  Instead, I get this:</p>
  
  <p><a href="https://i.stack.imgur.com/ataj6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ataj6.png" alt="Columns of color image"></a></p>
  
  <p>For some reason, this image is a series of bars rather than a solid color.</p>
  
  <p>What am I doing wrong? Is it the format of the numpy array?</p>