I would have expected the source file (in above example) to be some network location or possible \sysvol location as mentioned here.
http://woshub.com/copy-files-on-all-computers-group-policy/
then use the registry method mentioned here.
http://woshub.com/setting-desktop-wallpapers-background-using-group-policy/
in your example above both locations appear to be local on end user's pc C:\
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