How to change picture of "Drag to unlock your nook" screen that shows after boot? topic

How to change picture of "Drag to unlock your nook" screen that shows after boot, for NSTG with firmware 1.2.1 ?

This screen shows the photo of the park pines tree, and I want to change it but don't know where it needs to be changed.

I thought it was /system/media/screensaver/default.png as this was the same, but having changed that, I'm still seeing the tree pic.