After downloading the latest version Raspian and installing on a PI 3B+ I found I could not change the Desktop Wallpaper with right clicking and selecting Desktop Preferences.
From the command line:sudo pcmanfm
Navigate to /etc/xdg/pcmanfm/LXDE-pi
My folder had these two files
Change the line in each (it's possible the numbers have a precedence but I changed both)
Getting the .jpg file into the rpd-wallpaper folder requires command line: sudo pcmanfm. Copy and paste your file into /usr/share/rpd-wallpaper/
Just opening File Manager from the Raspberry Menu will not allow you to paste into that folder.