Error when trying to do an import of testbed

I would really like to write tests for Google App Engine using their python testbed library, but each time I try to import it using

from google.appengine.ext import testbed

I get the following error:

File "/Users/lhundeboel/Projects/busybeeapp/tests/test_something.py", line 1, in <module>
    from google.appengine.ext import testbed
File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/testbed/__init__.py", line 116, in <module>
    from google.appengine.api.images import images_stub
File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Conte开发者_C百科nts/Resources/google_appengine/google/appengine/api/images/images_stub.py", line 37, in <module>
    import _imaging
ImportError: No module named _imaging

Does anyone know what the _imaging module is, and how to I import it? I must admit, that I'm new to Python, so this may be something basic that I've missed.

It's the C-based part of PIL. Install PIL properly.