To implement the simple (for the user) captcha provided by Google one have to get an API key from Google to use it. Then the mechanism is quite simple: the Captcha code is an input of the form that will submit a value. The program receiving the form data will validate the captcha with a request to Google.
Inkscape gives the possibility to trace a bitmap object to a vector one from the menu Path > Trace Bitmap item, or Shift+Alt+B (see this page).
More interesting is the possibility to to the same from the shell, without the need of an interactive program. I loved potrace for this, available from the repository. Its input are BMP images, so imagemagik will become handy:
Installation is as easy as: git clone git://github.com/VitaliyRodnenko/geeknote.git
sudo python setup.py install
The latter will ask you to provide login credentials (press enter when asked for Two-Factor auth code, if you didn’t enable it) from the shell. While this is a good thing, unfortunately the authorization is not permanent, but “per session”.