Yes, you should be able to do so. Down at the bottom of your post, before the comments begin, you should see the 'Edit' option. Click that, insert your image, and save the updated post. It ought to update with the thumbnail in place.

When you do so, add the tag 'introduceyourself' to your list of tags as well. Many people and bots auto-upvote and respond to posts in that category, so you should see an upswing in views. :)