This article describes the recommended steps for adding an image to a post.
1.) Edit the post and scroll down to the "image" section within the post editor.
2.) Select "Add images..." and choose the photo you'd like to include with this post. After uploading, the screen will display an image token on the right-hand corner.
3.) If you do nothing with the token, then this image will be inserted automatically into the post. The first image is used for teasers by default unless the token is used in the body content. You can add the image token to the body of the post like this.
4.) Check out some additional image options here --> Advanced Image Configuration