I use J51Oxygen (Free) Template with Joomla 3.8.5 and JCE Editor. To insert an image from the Media folders I created in the Editor, I have to use the "Image" tab at the bottom of JCE Eeditor rather than the Icon. Once I have inserted the image I can then go to the icon in the editor and resize etc. If I do not do it this way I cannot see the files I uploaded via Media in the Joomla, only another set of folders. Why do I think this might be a J51Oxygen issue. Apart from doing lots of searching, I have three other sites that have the Beez3 template and they work fine.
Here is the link to the site:
http://halswellcommunity.net.nz/