Thanks for reporting
Files in the nodebb root directory are not served publicly, if you are putting them manually put them inside public/ or public/uploads folder. Then you can serve them with /assets/<filename>.jpg or /assets/uploads/<filename>.jpg
Where is the file? If it is uploaded to nodebb it should look like /assets/uploads/files/<filaname>.jpg
What url are you using for the image?
Can you try installing redis and setting it up as your session store. See if the error goes away.
having same issues, @julian do you guys have any tech support team who can help us? @baris