AttributeError: 'NoneType' object has no attribute 'read'

[Naereen]

Hi,
Sorry to bother again.
I tried your project, with some examples image, and it fails with this error:

Traceback (most recent call last):
  File "mosaic.py", line 346, in <module>
    main()
  File "mosaic.py", line 342, in main
    mosaic( input_image, image_stash, resolution, thumbnail_size, func)
  File "mosaic.py", line 280, in mosaic
    reply = Image.open( nn )
  File "/usr/local/lib/python2.7/dist-packages/PIL/Image.py", line 2549, in open
    fp = io.BytesIO(fp.read())
AttributeError: 'NoneType' object has no attribute 'read'

The script seems to start working correctly (I get the first 3 columns), but fails with a file error.