They are in the authui.dll in C:\Windows\System32 and if you have 64 bit system it will also be in C:\Windows\SysWOW64 both need edited. Use copy's of the authui.dll to edit.
They are in a Bitmap folder look at screenshot for bitmap #s there is 1 .png in the Image folder to. I edited them all to make sure I got it.
You will need to use a resource hacker of some kind to export/import the bitmap and png images and a image editor.
To properly change a system file you need to take ownership give yourself permission and rename the old file and then copy in the new one look at Post #6 of this thread will tell you how to do it Basic Networking Information
If you need help let me know.