CNN cartoon issues due to M51 picture

[bsipocz]

The M51 picture for the CNN cartoon brings up two low priority issues, one needs documentation only, the other a solution:

• the jpeg file requires Pillow as a dependency. Maybe the best solution is to convert is to png (the only issue is to make sure the result image is the same as what went into the book).

• using the current test and pdf generating mechanism (relying on extracting the code out to a temporary "somefile.py") is not working with the current solution for the file path of the image. Running the script directly works, so users shouldn't be affected by is.
  Copying a workaround from the astroML pickle_results mechanism is probably the easiest solution here.