Renamed JPGs immediately auto-revert to old name

I've seen the "save as" option get tricky if you manually add the extension to the existing file name rather than select that extension as an option. If you do this, the extension may show twice.

Extensions are NOT hidden - they clearly show after the '.' [and also within the name].

The likelihood of the the extensions being added to the name is miniscule - this is a phenomenon on someone else's PC.

that the problem you are seeing is on a remote PC and that you're not doing this directly from a local account. Is that correct? If so, that was not mentioned in the OP and it changes everything. If this is true, I wonder what the possibility would be of doing this test by someone in front of it.

' note read-only.' - do you mean 'not read-only' ?
The folder where these persistent names occur is C:\users\downloads\, so not 'like a CD'.
Are you suggesting the images should be copied/moved elsewhere before the attempt is made to change their names ?

That folder could have permission or rights issues.

To test that out, you could have copied one as a test. Why didn't you get a basic test done before we move to more advanced deep waters?

This happens on a 'remote' PC that I can not access at will. Will definitely try to move (else copy) a few image files with troublesome names to another folder, then try the rename.

Moving the items out of \downloads\ was what was needed to effect permanent name change. For the present I will settle for that as the needed solution; I realise the \downloads\ will get repopulated, and presumably the same problem will arise again.

While I can't guess how the folder came to start doing this, you can examine the rights, permissions to that folder and see if the user account has full rights.

Since that's on the web I'll defer to web articles. I know that some folk are taken aback on NTFS rights and permissions but it's an old topic. Well done.

https://www.google.com/search?q=show+ntfs+permissions+and+rights

Thanks for that reference. My relationship with NTFS rights is NOT a happy one - I don't know who to impersonate when I set permissions.

But I have had to get down to the command line and use ACL reporting tools to figure out odd permission/rights issues.

For most, we use our own account and might join the administrator group or add rights, change the permissions on the folder.

When I looked at properties of \DOWNLOADS\, I did not need to change any Permissions, because I saw that READ-ONLY had been 'ticked', and when I undid that, the problem disappeared.