To expand on vartec's answer to also explain how to use it...
Install webkit2png
The easiest way is probably to simply clone the github repo and run the setup.
mkdir python-webkit2png
git clone https://github.com/adamn/python-webkit2png.git python-webkit2png
python setup.py install
This requires python and git to already be installed.
For cygwin, this will add webkit2png as a command to the path. I haven't tested this for other terminals/OS.
Run it
Say you have your website in the current directory. (I had a html file that was using a css stylesheet - but there's no need to think about the css file.)  
webkit2png something.html -o something.png
Options
webkit2png -h informs us:
Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  -x WIDTH HEIGHT, --xvfb=WIDTH HEIGHT
                        Start an 'xvfb' instance with the given desktop size.
  -g WIDTH HEIGHT, --geometry=WIDTH HEIGHT
                        Geometry of the virtual browser window (0 means
                        'autodetect') [default: (0, 0)].
  -o FILE, --output=FILE
                        Write output to FILE instead of STDOUT.
  -f FORMAT, --format=FORMAT
                        Output image format [default: png]
  --scale=WIDTH HEIGHT  Scale the image to this size
  --aspect-ratio=RATIO  One of 'ignore', 'keep', 'crop' or 'expand' [default:
                        none]
  -F FEATURE, --feature=FEATURE
                        Enable additional Webkit features ('javascript',
                        'plugins')
  -c COOKIE, --cookie=COOKIE
                        Add this cookie. Use multiple times for more cookies.
                        Specification is value of a Set-Cookie HTTP response
                        header.
  -w SECONDS, --wait=SECONDS
                        Time to wait after loading before the screenshot is
                        taken [default: 0]
  -t SECONDS, --timeout=SECONDS
                        Time before the request will be canceled [default: 0]
  -W, --window          Grab whole window instead of frame (may be required
                        for plugins)
  -T, --transparent     Render output on a transparent background (Be sure to
                        have a transparent background defined in the html)
  --style=STYLE         Change the Qt look and feel to STYLE (e.G. 'windows').
  --encoded-url         Treat URL as url-encoded
  -d DISPLAY, --display=DISPLAY
                        Connect to X server at DISPLAY.
  --debug               Show debugging information.
  --log=LOGFILE         Select the log output file
Notable options are the setting of width and height.
Troubleshooting
Using cygwin, I encountered  webkit2png: cannot connect to X server :0.0.
To fix this (I had already performed export DISPLAY=0.0), I had to start an X-Server. On cygwin, this can be done by running startxwin in a second terminal. Make sure to install it first via the cygwin setup.