There are only 2 options for images in email, and you've mentioned both in your OP.
Assuming you want to keep hammering away at the Base64 embedded images technique, here are a few resources:
- http://www.campaignmonitor.com/blog/post/3927/embedded-images-in-html-email
- How to embed images in email
- http://www.phpeveryday.com/articles/PHP-Email-Using-Embedded-Images-in-HTML-Email-P113.html
In the Campaign Monitor link there is a support chart - not sure if Outlook 2013 is unsupported all together, or if it just blocks the image like a linked hosted one, but either way it looks like there is no solution for automatically showing embedded images in Outlook 2013.