Make google refresh the site preview screen shot
I believe you need to have your site working with Google Webmaster Tools to manually do this. So make sure you are setup with that first of all.
Login to the Google Webmaster Tools.
Choose your site from list of sites.
On the left, click "Labs".
Click "Instant Previews" which is a sub of "Labs".
That will do your root site, then type in urls and click "Compare" as needed.
This forced the cached screen shots to reset for me.
If you've made major changes you can also click "Diagnostics" (on the left above "Labs").
Click "Fetch as Googlebot"
Leave the text input after your site empty so that it fetches your home page.
Once complete, click "Submit to Index", then choose the radio for "URL and all linked pages", click "Submit".
Note that you can only fetch pages 500 times and submit them to the index 10 times.