After installing webkit2png in /usr/local/bin and running chmod a+x /usr/local/bin/webkit2png, I tried webkit2png --version and got the error Cannot find pyobjc library files..
Turns out webkit2png only works on OSX. See paulhammond/webkit2png/issues/77 and paulhammond/webkit2png/issues/9 for details. So far it seems the following are options to consider:
Webkit seems to be used by some if not all of these. Not sure if it has to be installed separately or if it included somehow.