Make using post thumbnails easier for everyday wordpressing.
In Settings → Media, click the plus sign to add a new post thumbnail.
Update the width/height and whether or not the post-thumbnail should be cropped to an exact size. (If the width or height is set to 0, that boundary constraint is not enforced). Make sure you click the "Save Changes" button so that the changes are stored in the database.
In the media editor, you can choose to insert a shortcode of a single picture with the "Insert shortcode" button.