The Shifting Desktop Background
Recent versions of Ubuntu (and the GNOME desktop environment) introduced a feature called “Light/Dark Backgrounds.” Depending on the specific theme you select, your desktop wallpaper might look like a bright, sunny landscape during the day, but automatically shift to a dark, starry night version of the exact same landscape when the sun goes down or when you enable Dark Mode. While this is a clever visual trick, it can be annoying if you explicitly chose the bright version of the image and want it to stay that way permanently, regardless of the system clock.
How to Force a Static Wallpaper
To stop the wallpaper from shifting based on the time of day or your Dark Mode preferences, you must avoid selecting the special “dynamic” image files provided by Ubuntu.
1. Open the main Ubuntu Settings application.
2. In the left-hand sidebar, click on Appearance.
3. Look at the “Background” section at the top of the window.
4. You will see a grid of default wallpapers. Notice that some images have a diagonal split down the middle (showing a light half and a dark half). These are the dynamic, auto-changing wallpapers.
5. To force a static image, click on any wallpaper that does not have a diagonal split.
Using Your Own Images
The easiest way to guarantee your wallpaper never automatically changes is to use your own custom photo rather than the Ubuntu defaults.
1. Click the Add Picture… button located above the wallpaper grid.
2. Navigate to your Pictures folder and select a standard `.jpg` or `.png` file.
3. Click Open.
4. Select your newly uploaded image from the grid.
Because your custom image is a simple, static file without the embedded XML metadata required for the Light/Dark transition, it will remain permanently locked to your desktop, no matter what theme you use.