Thanks, that was a good idea! I downloaded and extracted the archive from https://support.fairphone.com/hc/en-us/articles/360048050332-Manually-install-Fairphone-OS-on-FP3-FP3- (Step 4, “Download Fairphone OS (Android 10) onto your computer.”).
Then I used MTK Extractor 2.6.3 to unpack system.img. Inside I found WallpaperPicker.apk, renamed it to .zip and extracted it. The wallpapers are inside res/drawable-nodpi-v4 in this package.
However, these are the Android 10 wallpapers that are available on my Fairphone now 
So the next missing puzzle piece is the download link for an older version of Fairphone OS, which the support article doesn’t have. https://support.fairphone.com/hc/en-us/articles/360048139032-Fairphone-OS-releases-for-FP3-FP3- Lists releases, but doesn’t have downloads. Can anyone point to a download URL for Fairphone OS (Android 9)?