How do I add a thumbnail to the teaser of a blog post containing an image?
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.