In HTML mails image paths can be referenced relative to base URL which can be found in the HTML HEAD.
Explanation and example: http://www.w3schools.com/tags/tag_base.asp
<base href="<a%20href=" http:>" rel="nofollow" target="\_blank"\>http://www.w3schools.com/images/"\>; ![Stickman](stickman.gif)
eM Client (tested version: 6.0.21289.0) seems to ignore that, images referenced this way are not displayed, which is a major issue.