title: Add transparent picture over plot
body: <p>My python script below adds a picture (a generated rectangle on this simple example) and GPS track on a map generated with Basemap module.<br>
  Now I would like to make both track an rectangle transparent. No problem for the track via <code>alpha</code> kwarg but I cannot figure how to do it for the picture.</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from PIL import Image
  from matplotlib.offsetbox import AnnotationBbox, OffsetImage
  from mpl_toolkits.basemap import Basemap
  
  lats = [ 45, 15  ]
  lons = [ 0 , 100 ]
  
  fig = plt.figure( dpi = 300 )
  ax  = plt.subplot(111)
  
  myBaseMap = Basemap( projection='ortho', lat_0=lats[-1], lon_0=lons[-1] )
  myBaseMap.bluemarble()
  
  planeImg = Image.new('RGB', (600, 300), color = 'red')
  planeXY  = myBaseMap( lons[-1], lats[-1] )
  x,y      = myBaseMap( lons, lats )
  
  plt.plot( x, y, color='r', alpha=0.5, linewidth=3  )
  
  imagebox = OffsetImage( planeImg , zoom=.4 )
  
  ab = AnnotationBbox( imagebox, myBaseMap( lons[-1], lats[-1] ), xybox=( 0., 0. ), xycoords='data', boxcoords='offset points', frameon=False )
  ax.add_artist(ab)
  plt.show()
  </code></pre>
  
  <p>This code produces the picture below with a transparent line.
  <a href="https://i.stack.imgur.com/stDF0.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/stDF0.jpg" alt=""></a>
  Now I would like to make the red rectangle transparent in the same fashion.</p>
  
  <p>I tried to use <code>set_alpha</code> method on the annotation box and ax but didn't work.<br>
  Any ideas ?  </p>
  
  <p>Thanks.</p>