title: Image conversion in PIL, pgm file error
body: <p>When trying to do the following in the PIL python library:</p>
  
  <pre><code>Image.open('Apple.gif').save('Apple.pgm')
  </code></pre>
  
  <p>the code fails with:</p>
  
  <pre><code>  Traceback (most recent call last):
    File "/home/eran/.eclipse/org.eclipse.platform_3.7.0_155965261/plugins/org.python.pydev_2.6.0.2012062818/pysrc/pydevd_comm.py", line 765, in doIt
      result = pydevd_vars.evaluateExpression(self.thread_id, self.frame_id, self.expression, self.doExec)
    File "/home/eran/.eclipse/org.eclipse.platform_3.7.0_155965261/plugins/org.python.pydev_2.6.0.2012062818/pysrc/pydevd_vars.py", line 376, in evaluateExpression
      result = eval(compiled, updated_globals, frame.f_locals)
    File "&lt;string&gt;", line 1, in &lt;module&gt;
    File "/usr/lib/python2.7/dist-packages/PIL/Image.py", line 1439, in save
      save_handler(self, fp, filename)
    File "/usr/lib/python2.7/dist-packages/PIL/PpmImagePlugin.py", line 114, in _save
      raise IOError, "cannot write mode %s as PPM" % im.mode
  IOError: cannot write mode P as PPM
  </code></pre>
  
  <p>The code works fine with conversion to BMP, but JPG also fails.
  Strange thing is, a different file(JPG to PGM), works ok.</p>
  
  <p>Other format conversion. That is:</p>
  
  <pre><code>Image.open('Apple.gif').save('Apple.bmp')
  </code></pre>
  
  <p>works.</p>
  
  <p>Any idea why?</p>