Options for automating the capture of a screen shot of a web page.
Paparazzi!
Paparazzi! is an OS X application for capturing screenshots of web pages. It is AppleScriptable to automate the capture of a page:
tell application "Paparazzi!"
capture "http://www.google.com/"
repeat while busy
delay 1
end repeat
save in POSIX file "/Users/foo/Desktop/google.png" as PNG with icon
end tell
webkit2png