A workaround is to copy the desired wallpaper into /usr/share/backgrounds and rename it using one of the filenames of the default files found there:
$ sudo mv /usr/share/backgrounds/Delicate_Petals_by_lefthandgergo.jpg /usr/share/backgrounds/Delicate_Petals_by_lefthandgergo.bkp
$ sudo cp ~/Pictures/a-blue2.jpg /usr/share/backgrounds/Delicate_Petals_by_lefthandgergo.jpg
$ sudo chmod 644 /usr/share/backgrounds/Delicate_Petals_by_lefthandgergo.jpg
Now, opening System Settings -> Apperance should display your photo as Delicate_Petals_by_lefthandgergo.jpg
A workaround is to copy the desired wallpaper into /usr/share/ backgrounds and rename it using one of the filenames of the default files found there: backgrounds/ Delicate_ Petals_ by_lefthandgerg o.jpg /usr/share/ backgrounds/ Delicate_ Petals_ by_lefthandgerg o.bkp a-blue2. jpg /usr/share/ backgrounds/ Delicate_ Petals_ by_lefthandgerg o.jpg backgrounds/ Delicate_ Petals_ by_lefthandgerg o.jpg
$ sudo mv /usr/share/
$ sudo cp ~/Pictures/
$ sudo chmod 644 /usr/share/
Now, opening System Settings -> Apperance should display your photo as Delicate_ Petals_ by_lefthandgerg o.jpg