Problem is, there is no way to get those wallpapers from the stock phone. You need to have Full File System Access to get those pictures. I have downloaded leaked Windows Phone 8.1 Emulator image, but there were only Windows Phone 8 (GDR 0 to 3) wallpapers there (but 1080p quality, same ones that were presented on site above). So my request is only applicable to the interop-unlocked Lumias, if the phones like 920 has this location:
Also there would be nice if someone will provide maxed out lockscreen backgrounds from the current version of the Live Lock Screen app for Microsoft. If they are not protected somehow, all you need is to install the app to SD Card and then use @djamol method. Any Windows Phone 8.1 updated device should be up to do this. I tried to just move the app on my device from the phone storage to SD card, but then there is no Install folder I can copy files from. Doesn`t want to loose my background settings by reinstalling directly to SD card.