Question Ultra Power Saving Mode

Here is the thing

i have had changed wallpaper inside Keyguard.apk

set the permission to RW-R-R

move to root/system/priv-app second time flashed thru recovery

reboot

i did wipe cache/dalvik second time not

after that there is an error and i cannot use System UI, unless flashing stock Keyguard.apk and it will go back

question is how to change wallpaper of UPSM without having an error "android.systemui has stopped"

using S5 Sensation ROM