I collect nature and space images in two subdirectories on my hard drive. I
choose a new one every day.

The wallpaper images are located in /usr/share/backgrounds in Ubuntu, and is in
a different location for Kubuntu. By copying the two directories to
/usr/share/backgrounds, I select from the Main Menu:
System>Preferences>Appearance and then click on the Background tab, and then the
+ Add button. When the directory listing comes up in the window, I select
either nature or space, scroll down the new listing, and select the background I
want for the day. My favorite is from the space directory with a nice white
sandy beach, with a Palm tree slanted toward the blue-green water with an orb in
the sky like the planet Saturn - not unlike the beach in Contact which it
reminded me of, so I named the image Contact.jpg. :)

I may just write a script to automate the image for me everyday. Currently, I
have 258 images (nature-162, space-96).