Here's the version overview information from the site setup page.
Plone 5.0.8 (5018)
Python 2.7.6 (default, Oct 26 2016, 20:32:47) [GCC 4.8.4]
PIL 3.4.2 (Pillow)
This is the version installed using the vagrant set-up at https://github.com/plone/plonedev.vagrant.
ReferenceError: $ is not defined @@theming-controlpanel-mapper-getframe:125:3
which would seem to indicate something amiss with JQuery?
When I try what you finally got to work in your reply further down, i.e., delete (instead of rename) header-bg.jpg, before I rename my newly uploaded file (test1.jpg), it is displayed correctly:
But after I rename it to header-bg.jpg, it disappears:
I'm clearing the cache after every action based on the reply from pigeonflight below. If it helps, every time I clear the cache, in the Plone console, I see the informational message:
2017-08-10 14:46:12 INFO plone.app.theming.policy invalidating cache across all threads and processes