The only way I know of is to install Xposed, then LGG3TWB Lollipop and then check the appropriate box in the status bar tweaks area.
---------- Post added at 11:24 PM ---------- Previous post was at 11:06 PM ----------
A few notes to those who attempt to do this:
1) default_wallpaper.jpg need to be not only 2880x2560, but also 96 dpi. I was preparing my file by upsizing a jpeg within Adobe Photoshop, and PS defaults to 72dpi in its upsize command which caused a bootloop.
2) You can compress the JPEG to your desired level within PS without apparent adverse effects. I do not see the point of having a minimally compressed 2MB jpeg as a background when a medium compression JPEG at <500kb will do just fine, particularly given the size of the screen and the typical viewing distance
3) When pushing framework-res.apk back to /system/framework it is best to do in recovery (need to mount system as rw first). This allows you to push, chmod 644 if needed and the reboot. If you attempt to do this while booted to system the phone will reboot the second you replace the active framework-res.apk and if your permissions happen to be incorrect you are headed for a bootloop.
3) When you first reboot you will see the original lame desert drawing that LG seems so enamored of. Don't panic, you did it right, but the phone still remembers your old wallpaper. Just press and hold the back button to trigger the same bug and it will default to what you intended it to be.
Many thanks to KennyG123, who I hope does not actually listen to Kenny G