PBE: Images are overstretched

When inserting an image into a PBE-generated post (e.g. attaching a single image), the image is inserted to fit the `large` thumbnail size even when it is smaller the width.

Steps, assuming PBE is active with an e-mail created:
1. Send an e-mail to PBE attaching a smaller image (example: http://placehold.it/300x500 ).
2. View site.

Expected:
Image to load at full size (300x500 in this case)

Actual:
https://cloudup.com/c4Qi6ayXkCo
(using Twenty Ten, same with Twenty Fourteen)

HTML:
``

This occurs with Jetpack, works as expected with WordPress.com, perhaps due to the image classes being used for image sizing instead of set width/height attributes on WP.com.

Originally reported: http://wordpress.org/support/topic/images-too-big-posting-by-email?replies=3#post-5697902

Start with the PBE image-insertion flow and compare its generated HTML with the reported 300x500 example and the WordPress.com behavior. Confirm the fix by sending a smaller attached image and verifying that the post displays it at its original dimensions rather than the large thumbnail size.

Written by the indexing model from the issue text.