Do you have openssh installed? If not you should install it. Then use this guide. http://guides.macrumors.com/SSH_into_your_iPod_touch_(Windows)
After you ssh into it... you have to go to the var/root/library and search around for a img called "lockbackground.jpg/png/gif/etc". Then after you find it.... delete it.
It's now fixed. You can change the lock screen wallpaper again.