How to post a picture at the largest size (990px) in twentytwelve

You can grab the file url in the media library, then create a image tag in your post

Example:

<img src="http://development.wp/wp-content/uploads/2013/02/08.jpg" alt="Image Title" />

Or

you can insert from the add media widget and select full size from the drop down on the right of the media uploader.