How can I refresh the previews in Chrome's New Tab page?

I am using Chrome 6.0.472.0 dev and the New-Tab-feature. Some of my (sticky) previews were created in an "unlucky" moment, eg. superuser.com before I logged in

or GMail ... for whatever reason that preview is empty:

How can I force Chrome to refresh a particular preview?

One option is to go to the %USERPROFILE%\AppData\Local\Google\Chrome\User Data\Default directory and delete the Top Sites file (if using Chrome 14; in older versions it was called Thumbnails), but that will remove also the other settings of the New Tab page.

A better option is to get SQLite and reset only the thumbnail data. Download the SQLite binary, unpack it to the above mentioned directory, and run the following command:

sqlite3.exe "Top Sites" "update thumbnails set thumbnail=null"

After that the icons on the New Tab page will be broken, but they will be updated after you visit each of the pages once.