Wrong folder information for the Wilber_Construction_Kit.
According to the gimp-first-steps.html page (https://docs.gimp.org/2.10/en/gimp-first-steps.html):
The Wilber_Construction_Kit (in src/images/) allows you to give the mascot a different appearance.
In reality, there is no src/images/ folder and the actual location of this file is "docs/Wilber_Construction_Kit.xcf.gz".
You may check on the source code: https://gitlab.gnome.org/GNOME/gimp/tree/gimp-2-10/docs