Default Go Locker theme: Changing wallpaper image in apk

Hi. Not sure if I'm posting in the right place... but here:

I've just paid ~$21 for the Go Locker VIP Privilege so I can download unlimited themes but I am very disappointed by the fact that we can't change themes' wallpapers. It doesn't fit at all with the look of my (themed) launcher.
I have replaced the wallpaper files for my own using the WinRar method and it worked. But as soon as I restart the phone, the modded theme is gone and Go Locker set back the default one.
Theme icon is still on the home screen but it says it's not installed.

Is there a way around this? Signature problem? N00b here. Got my S5 6 days ago, had an old flip cellphone (and a pager!) before.