how to reload/refresh Gravatar images

I imported many comments from another database and now they all display with my own photo as the gravatar. The email addresses of the commenters are intact. I was going to post the question here but I discovered that what happened is during my import, the user_id column of the wp_comments table had been populated with my own user_id. Updating this to 0 causes the gravatar to be fetched normally by looking up the email address supplied by the commenter. So this is not a quesiton, just a post that I hope helps someone in a similar situation.