I experience more or less the same:
No matter what file I upload, the image
https://i.stack.imgur.com/gGfsT.png is displayed:
I helped myself as follows:
I browsed to the page
https://imgur.com/upload and uploaded my image-file there.
The file got uploaded and the uploaded image was displayed.
I clicked the image with the right mouse-button.
A context-menu opened where by clicking with the left mouse-button I could select "copy image address".
This way I had the URL of the image stored in the clipboard.
I switched back to TeX-LaTeX-StackExchange and created the link to the image manually:
Pattern in the window for editing is:
- Within the text:
[![enter image description here][⟨number of link⟩]][⟨number of link⟩]
- At the bottom:
⟨space⟩⟨space⟩[⟨number of link⟩]:⟨space⟩⟨URL⟩
⟨URL⟩ is the URL stored in the clipboard.
⟨space⟩ is a space-character (code-point number 32(dec)=20(hex) in Unicode).