I'm a fan of www.thumbalizr.com. All you have to do is get the direct URL and plug it in and this site does the rest. The only problem I have found is that it tends to cut off the bottom of the last row - so if your item is in the last row, this tool won't do you any good.
So, I've found a great post on how to take screen shots for you. It's from a website called The Curious Pug and it gives you the straight deal. You can find it here: http://curiouspug.blogspot.com/2009/12/tutorial-3-how-to-take-screen-shot-of.html It should answer all your questions, but if not, you can ask me and I will do my best to sort it for you.