Print the whole webpage in a single image file

It only does PNG, but Firefox has a way to capture the whole page built in: Shift-F2 brings up a command prompt, which includes a screenshot command. For instance, screenshot –clipboard –fullpage as I was writing this answer produced http://imgur.com/tnplKPE.

If you want something designed to be automated, phantomjs has page.render() which takes a filename and an optional options object, with format and quality entries; the example given is

page.render('google_home.jpeg', {format: 'jpeg', quality: '100'});