A couple of methods come to mind:
You could take a picture of your phone with another camera.
You could do a screen capture of you phone screen using built in functionality of the Android operating system. The method of doing a screen capture is different depending on which version of operating system you have. On my phone, I press and hold the POWER + HOME buttons at the same time and a screenshot is capture and saved to my SD card. Some older version of the Android operating system didn't have the screen capture functionality built in so depending on your version of the operating system, it may or may not be an option for you.
I have written a few Android apps before. I just checked the Android API's and it looks like I could write an app to retrieve the wallpaper image from your system memory and save it to a SD card if you have an SD card in your phone. The app would just read the image and wouldn't change it.
Also, do you have an SD card that is working now?
I understand how frustrating it is to lose an important image and would like to help you get it back if I can.