I've looked through various such tools, but none of them are to be used in Linux:
Try wkhtmltoimage from wkhtmltopdf (check the downloads) Its great tool and ASAIK works on windows.
KHTML2PNG does that for Linux, using the KHTML rendering engine. We're using it in headless mode on servers.
There is an Add-on for Firefox called Screengrab that places an icon in the browser status bar. Not command line, but either Linux or Windows, and handy.
We use wkhtml2pdf which is a static build of the Webkit rendering engine (as used by Safari, Chrome et al.) and generates a PDF. Not quite a JPEG/PNG screenshot, but it serves a different purpose for us. (As an added bonus, you can still copy the original text as-is.)
It works on Linux, OS X and Windows.
There is an addon for firefox called Pearl Crescent that has command line access:https://addons.mozilla.org/en-us/firefox/addon/pagesaver/?src=collection&collection_id=23ccef22-af27-4a82-806a-568db8935e21