To change the Wallpaper, (If you downloaded the iMIUI Wallpaper Package from Cydia or want to use one of the stock iOS wallpapers)in iFile go to Library > Wallpaper > iPhone > Copy the file you'd like to use.
If you have another image you want to use simply locate the image in iFile, copy it then follow the steps below.
Again in iFile navigate to Library > Themes > iMIUI LS Clock Widget.theme once in this folder delete the file names "LockBackground.png" and then paste in your new wallpaper image and rename it to "LockBackground.png)
Re-spring your device and that's you done.