After mulling this over and searching the web for a solution I submitted a case to Autodesk and as usual the Autodesk support team (in this case it was Lance Coffey who has also helped me out in the past) promptly replied with the answer.
So here's what you need to do if your family "thumbnail" icon dose not match your family (direct from Autodesk support!):
1. Open the family file.
2. Save the file to a new name, and click Options on the Save As dialog window.
(Actually I just overwrote the original file and it worked fine)
3. Select the Source view that you want to use for the thumbnail preview, and check the "Regenerate if view/sheet is not up-to-date" box.
(This is the important part!)
4. Save the file, and check if the thumbnail is updated.