I am running 8 in a VM so my results may be different than an actual install. let us know
DVD\sources\$OEM$\$1\ProgramData\Microsoft\User Account Pictures replace all pictures in that folder with same size guest.bmp guest.png user-200.png user-40.png user.bmp user.png user.bmp is also found in MountDir\Windows\WinSxS\amd64_microsoft-windows-usertiles-client_31bf3856ad364e35_6.2.9200.16384_none_825f8ac8a2b29ff7\user.bmp
Depending on your version or language the numbers after _none_ may be different. I checked a French release dvd and it is same
Windows\Web\Screen\img100.png is Default LockScreen image. the cartoon looking tower add DVD\sources\$OEM$\$$\Web\Screen\img100.png through img105 1920x1200 image You can change that one and all the rest if you want then you are able to swap it in Settings on right mouse sweep. At bottom 'Change PC settings' and it comes up on lock screen. Small image there will still be the default but it loads your img100.png Now each time you change the resolution it will use your pic.
EDIT: Windows.UI.Immersive.dll contains the small images
This post has been edited by maxXPsoft: 12 November 2012 - 12:01 PM