title: Why can't PIL modify gray-scale BMP image data?
body: <pre><code>im = Image.open("grayscale.bmp")
  data = im.load()
  data[1,1] = 1
  </code></pre>
  
  <p>It reports "ValueError: image is readonly". Only 8-bit grayscale bmp image will raise this exception. Why?
  BTW, I cannot get the palette from 8-bit grayscale bmp image. But according to the BMP specification, there exists palette. Why?</p>