Are you suggesting that this issue haven't reported at Joomla issue tracker?
Yes, but when I check again this issue, I found out that this might be not an issue from the Joomla, it supposes to show this kind of code into the image path (e.g. images/apple-touch-icon-57.png#joomlaImage://local-images/apple-touch-icon-57.png?width=57&height=57)
Perhaps you can report this issue to the Joomla issue tracker and see how it goes.
https://github.com/joomla/joomla-cms/issues
By the way, I've applied some temporary fixes to the following files on your site:
JoomlaFolder/plugins/system/helix3/core/helix3.php
JoomlaFolder/templates/office/html/com_easysocial/helpers/html/login.php
Can you give it a try upload a new background login image and see if it displays fine?