What size is the full Start screen image, including edges?

[DCTF]

Can anyone tell me what resolution my images need to be for a Lumia 920 wallpaper? I know it's a 768x1280 screen, but the phone stretches images of that size beyond the edges of the screen, in order to make the parallax effect when scrolling.

I'm trying to get an accurate position for some elements on WP10, and there isn't even the moveable crop area unlike WP8.1 so I have to try and get it right in Photoshop at square one. Thanks!