Outlook only shows placeholder for the image (apparently cannot decode it), and ticket history displays 'Message body is not shown because it is too large' (see picture below).
This is likely the key issue. There is a config value that defines the max size that will be displayed inline. As noted in the docs, you can set this in individual preferences or in the main RT_SiteConfig.pm, so you can test it easily by changing the user preference and re-checking your ticket.
http://bestpractical.com/docs/rt/latest/RT_Config.html#MaxInlineBody --
