Greetings asiangolfer,
My interests were peaked after a simple search did not provide the answer. The way I've found to get them might be a little drawn out, but it worked for me.
First I use root explorer. I don't know if other programs will let you do the same things that it does, so you may have to try a few different ones if you don't want to buy the app. Anywho...
1. First navigate to the data/app folder on your SD card.
2. Locate the particular theme that you want to get the wallpaper from.
3. Select the theme. On my setup, package installer pops up a window and asks what to do..install,view or cancel. Click view
4. Select the res folder
5. select the drawable-hdpi folder
6. Find the default_wallpaper.png
7. Long press on it and the option to extract comes up. Select extract and the file is moved to the extracted folder on my SD card.
8. From there set wallpaper with your app of choice.
9. If you plan on extracting more than one file. You have to rename the files in your extracted folder or else they will over write one another.
Like I said, this way is a little drawn out. I don't know if there is a easier way or not.
Hope this helps