Graphicriver utilizes a "Thumbnail hover popup" technique on it's pictures.
Whenever you hover on the thumbnail a larger image is displayed as you can tell on my small screenshot below:
In my opinion they're by using this:
How do i accomplish this on my small wordpress blog?
Any ideas men?
Thanks ahead of time
You will need to understand how to use jQuery to do this effect. Then stick to the plugin's instruction here
You may even search in to the demo source code to determine the way the wordpress plugin used.
Sorry, no step-by-step walkthrough since it is such a long time and based on your wordpress setup.
Best of luck friend.