Revit doesn’t provide API to export/extract thumbnail from Revit files, especially Revit family files. However, there always is workaround in the world.

There is a very good article talking about this with detail, here is the link:

http://redbolts.com/blog/post/2008/12/01/Getting-your-Revit-thumbnails.aspx

If this isn’t work for you. There is another way: open the Revit file, export a bitmap then close it and use the bitmap according it. If anyone is interesting in this, I will write article later on this topic.