If your Mac reverts your desktop images back to the defaults after a restart try the following

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$ rm -rf ~/Library/Preferences/com.apple.desktop.plist
# This file may or maynot exist
$ rm -rf ~/Library/Preferences/com.apple.desktop.plist.lockfile

Reboot. The desktop images will be the default, so go ahead and reset them to what you want. Restart and they should remain.