Hello, it's both! Windows 10 Mail isn't loading the images at all, and Outlook 2013 is not loading them with their full width. Outlook 2010 is rendering them correctly, though. I am yet to test on Outlook 2016 and Outlook 360.
As for your suggestion, I'm not sure I understand. The code snippets in your comment look identical save for missing < > symbols in the first snippet. However my code already does look just like what you put in the second snippet. Or am I missing something? I might not be seeing the subtle change you suggest