Is it possible to export the current iPhone wallpaper?
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/&lt;youruser&gt;/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
chmod +x ~/Downloads/img-separator.sh
BACKUP=$(ls -t . | head -n1)
~/Downloads/img-separator.sh "$BACKUP"


What those four lines do, for non-programmers:


"moves" you into the Backups directory
Makes the script I wrote above "executable" (able to be run)
Gets the name of the newest backup
Runs my script, telling it the name of the backup to process


And here is a description of what the script does.