Posted 30 October 2012 - 19:49
It's easier to just change the file out from the original .WIM file before you install, but once you've installed this is where the file lives (and it's not easy to get to, as you found out, as Windows has really locked down those locations). Until someone writes a tool to do this automatically (Microsoft or otherwise), this is the way. I generally recommend changing the files out on the original source install.wim file, of course, as it's a heck of a lot easier (although you still need to take permission of the folder itself).
If you use dism to mount the WIM file, the files are located in <mount dir>\Windows\Web\Screen, and named img100.png - img105.png. Obviously, whatever is named img100.png gets to be the default lock screen when you install Windows from this WIM.