How can I get google search results to change my thumbnail?

ChancyRat

Sorry, I don't know where this belongs. Google search results always show a thumbnail of my homepage, but it's a quite outdated one (from Gold to Summer). I have changed themes since then. What can I do to get google to update their thumbnail? It's been well over a year so I doubt "wait for them to crawl you" is the fix.
thanks.
www.joinrats.com