I have cm12.1 and make simple script with Tasker, that change screen wallpaper every time, after screen unlocked. It start by Event screen unlocked, first step-copy file with random name from folder with images to temp folder with changing name to keyguard_wallpaper. Second step-move file from temp folder to the path from OP with root,( copy file without temp folder with root not working for me). And first time i set permissions to 777, not changing owner and group, and it works very well. But after 2 days it suddenly stop working-file moved correctly, but wallpaper not changed, it change only after reboot... May be it's way to change permission from tasker? (I think only permissions may stop work)
Ps It seems i solve not working-in the file keyguard_wallpaper_info.xml was set wrong width of wallpaper-1920 instead of 1080. When i set it to correct value, all works with permission rw-rw-r and owner root and group sd-card-write. And if somebody need, i can upload my tasker task for changing wallpaper every time after unlock,so every tome you see new wallpaper