Can't change photos in Preview.app – permission problems only in ~/Desktop After upgrading to Sierra I can't change any document with Preview if it is saved under ~/Desktop. I get the error "The original document can't be chnaged, so a duplicate with your changes has been created."
However, if I copy the same file to my Documents or Dropbox folder I can edit it.
The folder permissions for ~/Desktop my Documents folder are the same:

drwx------+  54 myname  staff   1836 Oct  4 10:41 Desktop
drwx------+  26 myname  staff    884 Oct  4 10:40 Documents


Permissions for the file is 

-rw-r--r--@   1 myname  staff      247097 Aug  9 09:45 picture.jpg


Even if I change the picture and Desktop folder to 777 I can't edit it.
Right click on the Desktop icon in finder, show info. Is the the folder selected as 'Locked' for some reason (little padlock bottom left corner of icon in Info window)?

Expand the 'Sharing and Permissions' section, should say 'custom access' and the privilege should show Read &amp; Write.

Click the little padlock bottom right of Info window, enter admin details and click the little cog, and select 'Apply to closed items'.

Could also open Disk Utility, select the drive, click 'First Aid' and check the volume for errors. This last may not have any effect, and should come back showing as all good. The 'Repair disk permissions' will be completed automatically whenever you run any software updates and does not require you to do it manually any more.