Have you had issues where your Revit 2012 thumbnails have disappeared, even though Windows is set to display them?
This often happens after uninstalling older versions of Revit, in particularly Revit 2010. Therefore to resolve this, open up a command promote & copy & paste this line into the command line.
regsvr32 "C:\Program Files\Autodesk\Revit Architecture 2012\Program\Revit.Thumbnail.dll"
You will need to be a administrator to do this, but after you have done this, reboot your machine & you should find thumbnails will return.