The '..' prefix seems to be some TinyMCE issue (or perhaps related to the way it is customized for Tersus integration), but in any case, the default popup provided by TinyMCE when clicking the Add Image icon, is missing major functionality (selection/upload of images).
This is the reason we override the default popup with our own implementation in tersus.web (the application providing this site). Have a look at the tersus.web/Reusable Models/HTML Editor/Insert Image/<Insert Image> button, for a practical example.
Of-course you can also have a look at the default integration of TinyMCE with our client, in [Studio root]/plugins/tersus.appserver_[version]/DefaultWebapp.
For best results, use the Firefox browser..