The images are saved in
~/.cache/plasmashell/plasma_engine_potd $ ll
drwxrwxr-x 2 dkb dkb 4096 Dec 20 15:43 ./
drwxrwxr-x 9 dkb dkb 4096 Dec 20 12:11 ../
-rw-rw-r-- 1 dkb dkb 3540733 Dec 20 15:43 apod
-rw-rw-r-- 1 dkb dkb 3462109 Dec 20 12:11 bing
-rw-rw-r-- 1 dkb dkb 41607252 Dec 20 14:30 wcpotd
~/.cache/plasmashell/plasma_engine_potd $ file *
apod: PNG image data, 3049 x 1033, 8-bit/color RGB, non-interlaced
bing: PNG image data, 1920 x 1080, 8-bit/color RGB, non-interlaced
wcpotd: PNG image data, 7963 x 4886, 8-bit/color RGB, non-interlaced
To use these images, copy them over to the folder in which you store your wallpapers and modify the filenames to have
.png as the file extension. This step is necessary for the GUI to list them.
By the way, a deleted answer by miharix linked to a site where it's possible to view previous Bing wallpapers along with a brief description of each wallpaper.