How do I add a thumbnail to the teaser of a blog post containing an image? I am using Drupal 7 and have added an image field to the blog content type.  The image comes in fine when creating the blog post, but when viewing the teaser of the blog entry in the list of user's blogs, there is no thumbnail in the teaser showing that there is an attached image.  Is this possible to have? How do I do it?
Go to display settings for your content type ('blog post'), and then select teaser ex.:

/admin/structure/types/manage/article/display/teaser

From here you can control which fields will display in your teaser (or full, or other defined formats). Your image field is most likely hidden for your teaser, you'll want to have it displayed and style accordingly.