There are a few options to take screenshot of a webpage or save a webpage as an image on the disk via the command line. This can allow you to create a thumbnail of a web site easily by one command on the terminal.
Using chrome to save a webpage as an image:
chrome --screenshot=google.png --headless --window-size=1024,768 https://www.google.com/
Using cutycapt to screenshot a webpage:
Using webkit2png to create site thumbnail:
You can also use Midori browser