JPG thumbnails were not being displayed in nautilus. After looking around I found Bug #515728 on the Debian bug list. I did two things suggested:
sudo cp /usr/bin/update-mime-database.real /usr/bin/update-mime-database and sudo rm -r ~/.local Not sure which of those made the difference but jpg images are now correctly previewed as thumbnails in nautilus. I suspect it was deleting the .local folder.