Xubuntu default wallpaper not respected by Xfdesktop 4.20
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# Issue Description
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The default Xubuntu wallpaper is not loaded in Xubuntu 25.04 with Xfdesktop 4.20.0. Reference downstream issue: https://bugs.launchpad.net/ubuntu/+source/xfdesktop4/+bug/2092806
# Steps to Reproduce Issue
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1. Download and boot the Xubuntu 25.04 daily ISO: https://cdimage.ubuntu.com/xubuntu/daily-live/current/
2. Observe that the Xfce default wallpaper is loaded by Xfdesktop.
3. Open the desktop preferences. Observe that the Xubuntu default wallpaper is selected.
# Expected Behavior
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The Xubuntu default wallpaper should be loaded when Xubuntu 25.04 is booted. The preferences dialog correctly identifies the configured default, so there appears to be an issue with the background loading itself.
# Additional Information
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Screenshot demonstrating the difference between the loaded desktop image and the configured one:

# Environment & Versions
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* OS distribution and version: Xubuntu 25.04 daily ISO (2025-01-01)
* Overall Xfce version: Xfce 4.20
* xfdesktop version (git hash if not a release): 4.20.0
* Windowing system (X11/Wayland): X11
* Compositor and version (if Wayland):
* GTK Theme: Greybird
* Icon Theme: elementary-xfce
* UI Scale Factor: 1.0
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