So, has anyone suggested re-registering any.

It contains a ImagingDevices.exe file and 4. In the Program Files folder (x86) and normal, there is a Windows Photo Viewer folder. Maybe the driver for you video card, or some utility associated with it has altered the normal behavior. You might open the Default Programs, Set Associations and check some of the file associations and see if Windows PhotoViewer is listed or something else. Not sure why it would have trouble getting access. It also opens a file in the temp section of your AppData\Local folder. You might check the Event Viewer to see if any Errors or Warnings show up around the time you try to open a picture. Also, an Anti-virus will look at the file first. It checks for any 'suppression' keys or open only in app keys, or only in Browser keys and permissions. Photoviewer looks at many things before it opens pictures.

But since you have already done that and it found no problems, I suppose we will have to look somewhere else. When I see Windows doing something out of the ordinary for a basic Windows utility, I suggest running the SFC /scannow utility.