Fix issues with the image mode

I got a bunch o' these overnight:
```pytb
Traceback (most recent call last):
File "/app/.heroku/python/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request rv = self.dispatch_request()
File "/app/.heroku/python/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request return self.view_functions[rule.endpoint](**req.view_args)
File "/app/.heroku/python/lib/python2.7/site-packages/newrelic-2.26.0.22/newrelic/hooks/framework_flask.py", line 40, in handler_wrapper return wrapped(*args, **kwargs)
File "/app/downsample.py", line 29, in service image.save(dest, format='jpeg', quality=random.randrange(3, 6))
File "/app/.heroku/python/lib/python2.7/site-packages/PIL/Image.py", line 1663, in save save_handler(self, fp, filename)
File "/app/.heroku/python/lib/python2.7/site-packages/PIL/JpegImagePlugin.py", line 478, in _save raise IOError("cannot write mode %s as JPEG" % im.mode)
IOError: cannot write mode P as JPEG
```