Lost wallpaper photo!

If you set a certain picture as a wallpaper and then delete the photo from the photo library, the photo can still be seen in the Settings App under Settings --> Wallpaper, but that does not mean that the photo is still existent as a JPEG-File on the device. I would just screenshot the lock screen and remove / crop the time and status bar from that screenshot.

Have you looked for it in your Photo app on your iPad? Did you arrange photos in folders at all? If you never created a folder of say, family photos, you'll have to look through the photos in your all photos folder for the picture.

If the photograph can be found in Settings > Wallpaper then it is on your phone. You just need to see where it is. Check every folder you have on the iPad. It's there somewhere.