windows 7 - A single file has become unusable, unmodifiable, undeletable. Cannot take ownership

A single file, at /MyFolder/images/file1.png, has become, suddenly, a god entity as I've never seen one before. I'm the only account on the PC, full admin rights. UAC turned off always, ever since the beginning. What has failed:

• Windows Explorer -> Properties -> Security:


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  • "You do not have permissions to view or edit this object's permission settings"
  
  
  


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  • Advanced -> Owner: Current owner: Unable to display current owner
  
  


• takeown /f file1.png /r /d y: ERROR: Access is denied.


• icacls file1.png /grant administrators:F /T: file1.png: Access is denied. Succesfully processed 0 files; Failed processing 1 files.


• net user administrator /active:yes in an elevated command prompt, switching to Administrator user, trying to take ownership through Windows Explorer & repeated above steps.

Whatever I do, I cannot retake control of this file. What have I missed, what can I do?