help please…

I’m working on a new WP theme foe tutorials index like good-tutorials.com
and I want to add the preview image in the laft side of the content&title…
here is a sample:

https://img204.imageshack.us/img204/5361/sampleca2.gif

I want a option in the post writing erea for adding this kind of preview image..

how should I do it? should I add a plugin? if yes, which?

thanks ??