Rather than post big photos, I will post a description.
These days, I have reverted to solid color backgrounds mostly in the medium-dark blue and medium gray ranges. It helps me distinguish between privileged and non-privileged accounts on the same host and also between hosts. (Privileged/Admin - gray, non-privileged working account - blue.)
Before I switched to solid colors, I used images. I looked for images that make Desktop icons easy to see. Here are some of the images I used and may use again someday. You are on your own to search for them:
- Many astronomy photos. That backlit view of Saturn taken by the Cassini orbiter is particularly good.
- A Good Day in Harlem, 1958 - An amazing gathering of jazz musicians from the 1950s.
- Solvey Conference, 1927 - An amazing gathering of physicists from the 1920s.
- Studley Tool Chest, late 1800s - The workmanship is exceptional in every way.
- Two "merged" images of Earth, spread like a map projection - One in daylight and one at night.
Good luck!