I ran:

python imagetransfer.py 'File:matplotlib_screenshot.png' -tofamily:lwiki -tolang:en -family:commons -lang:commons

and received the output:

/var/www/localhost/htdocs/w/core /var/www/localhost/htdocs/w/core
0. Found image: [[File:Matplotlib screenshot.png]]
== {{int:filedesc}} ==
|Description    ={{en|1=A screenshot showing [[:en:Matplotlib|Matplotlib]] plots of one 3D ''plot_surface'' graph and a polar bar graph resembling the Matplotlib logo together with the graphic's python source code opened in a text editor.}}
|Source         ={{own}}
|Date           =2014-05-01
|Author         =[[User:Geek3|Geek3]]
|Permission     =
|other_versions =
== {{int:license-header}} ==
[[Category:Screenshots of software]]
[[Category:Debian screenshots]]
[[Category:Created with Matplotlib]]
The image is already on Wikimedia Commons.

Which I found confusing. I have InstantCommons enabled on my Wiki, if relevant. No File:Matplotlib_screenshot.png page or file exists on my Wiki (except for the mirror version of the page that exists due to InstantCommons). Transferring from the English Wikipedia (namely transferring File:Matplotlib_logo.svg) gives a different message (this one is an error message): 

ERROR: Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/pywikibot-2.0rc1.post2-py2.7.egg/pywikibot/data/api.py", line 1927, in submit
    body=body, headers=headers)
  File "/usr/lib64/python2.7/site-packages/pywikibot-2.0rc1.post2-py2.7.egg/pywikibot/tools/__init__.py", line 1248, in wrapper
    return obj(*__args, **__kw)
  File "/usr/lib64/python2.7/site-packages/pywikibot-2.0rc1.post2-py2.7.egg/pywikibot/comms/http.py", line 244, in request
    r = fetch(baseuri, method, body, headers, **kwargs)
  File "/usr/lib64/python2.7/site-packages/pywikibot-2.0rc1.post2-py2.7.egg/pywikibot/comms/http.py", line 401, in fetch
  File "/usr/lib64/python2.7/site-packages/pywikibot-2.0rc1.post2-py2.7.egg/pywikibot/comms/http.py", line 318, in error_handling_callback
    raise request.data
ReadTimeout: HTTPConnectionPool(host='localhost', port=80): Read timed out. (read timeout=30)
WARNING: Waiting 5 seconds before retrying.

which was repeated until the last line became: "WARNING: Waiting 40 seconds before retrying". After this was the following error message: 
We got the following warning(s): exists: File Matplotlib_Logo.svg already exists.
Do you want to ignore? ([y]es, [N]o): n
Upload aborted.

But the transfer did occur successfully (i.e., File:Matplotlib_Logo.svg now exists). 

Thanks for your time,