I found this post by Barak Korren. Barak wrote a short nautilus script in python to allow the easy deletion of a thumbnail in Nautilus. Here’s a step by step guide to getting it working.

Download this file and put it into your ~/.gnome2/nautilus-scripts directory. The script is by Barak, I uploaded a plain text version here to make it easier to download. Make the script executable, you can run chmod +x ~/.gnome2/nautilus-scripts/delete_thumbnails.py in a terminal to do this. Now go to that directory in Nautilus, and you’re in business.

To test, right click on a file with a thumbnail. You should see a new menu, Scripts, under which you’ll see “delete_thumbnails.py”. Click that option and the thumbnail will be deleted. Press F5 to reload the folder in nautilus, and you should see a new thumbnail generated.

Thanks for a such a handy script Barak.