Okay, after a lot of testing, I have discovered this:
Don't cut images to your native resolution. The OS is reintroducing parallax scrolling, which means your wallpapers need to fill a gutter partway off the bottom of the screen into which the view can move. If the phone is given a 1080p wallpaper, it will stretch it to a large size to fill all this space, making a blurry image. The screen will also use the rightwards space to scroll a little to the right when entering the app list menu.
So, your wallpaper must be 1297 (width) x 2300 (height).
Example image: here's another Tomer Hanuka image I was trying out. http://i.imgur.com/jkYUcjb.jpg
I don't know if there's a proportional relationship here which remains true for other screen resolutions.
Current L930 layout (tiles continue off the bottom of the screen for the "reel" effect I was using last time):