If you are looking for a wordpress plugin that will automate the job for you than "Simple Website Screenshot" does a fantastic job. You can specify the thumbnail size etc.

http://www.digg-it.info/wordpress-simple-website-screenshot-v-0-5

We use it on our own website (http://productivewebapps.com) if you'd like to see an example of it in use! (Disclaimer: I am the owner of this site)