Hi Ed,
This has nothing to do with the editors.
It is mostly about the media manager and secondly about the background option in any custom html module.
Conditions:
1. If slave media manager image path is images, the media manager links to master/images and path becomes var/www/images.
2. If slave media manager image path is mutlisites/slave/images, custom html module background-image style looks for mutlisites/slave/images/img.jpg
Screenshots below.
With var/www/ appended when looking at the media manager:
There's a new feature in 2.5 custom html module where one can select a background image via the media manager.