Fedora 39

From a bash prompt:

su - 
cp DesiredImage.png /usr/share/backgrounds/f39/default/f39-01-day.png
cp DesiredImage.png /usr/share/backgrounds/f39/default/f39-01-night.png

Fedora 38

From a bash prompt:

su - 
cp DesiredImage.png /usr/share/backgrounds/f38/f38-01-day.png
cp DesiredImage.png /usr/share/backgrounds/f38/f38-01-night.png

That is it!