As an expansion to Macjack's answer, here's the process I just followed to retrieve the lock screen wallpaper on my phone.
Looking in the /Users/<youruser>/Library/Application Support/MobileSync/Backups
folder is effective but very time consuming, especially if you have a lot of files/images in your backup (my phone a 16GB iPhone 5 that is pretty full had nearly 7000 files in there). However, the majority of them were not photos for me.
So I wrote a small script to extract the photos and rename them so they have the proper file extension. This makes it possible to use the image carousel preview for reviewing the images quickly.
Get the script here. (right click and 'Save link as' and save into your Downloads
folder)
Now, open Terminal (type 'terminal' into spotlight). Then copy and paste this into the terminal window:
cd ~/Library/'Application Support'/MobileSync/Backups ~/iPhone-Backup-Copy
BACKUP=$(ls -t . | head -n1)
cp -rv "$BACKUP" ~/iPhone-Backup-Copy
cd ~/iPhone-Backup-Copy
chmod +x ~/Downloads/img-separator.sh
~/Downloads/img-separator.sh "$BACKUP"
What those six lines do, for non-programmers:
- "moves" you into the Backup directory
- Gets the name of the newest backup
- Copies the newest backup directory into a place with looser permissions
- "moves" you into that new backup copy
- Makes the script I wrote above "executable" (able to be run)
- Runs my script, telling it the name of the backup to process
And here is a description of what the script does.