Location of default Lubuntu desktop wallpaer:
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You may need to get here to replace the file if needed diffrent wallpaper or lockscreen background, or just find the origonal.
This is for lxde.

/usr/share/lubuntu/wallpapers


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Source: https://askubuntu.com/questions/444306/location-of-default-lubuntu-desktop-images#444308
Date: September 2021.