- Backup your iOS device to your computer (not iCloud) through iTunes
- Download iPhone / iPod Touch Backup Extractor from http://supercrazyawesome.com/
- Run the app, and click "Read Backups"
- Find the backup you just made
- Make an empty directory
- Scroll all the way down and highlight "iOS Files"
- Extract the iOS files to the directory you made
- Visit https://www.dcode.fr/cpbitmap-format
- Click "Choose File"
- Inside the directory you created, find the iOS Files/Library/Springboard directory.
- Upload HomeBackground.cpbitmap. That will invoke a download the image in PNG or JPEG format.
- Upload LockBackground.cpbitmap for good measure. That will invoke a download the image in PNG or JPEG format.
These instructions owe a debt to http://www.iclarified.com/entry/?enid=4974