Where can I change the 'flower' image that is shown when users are logging on to the domain in Windows Vista and Windows 7?
The Account Picture can be changed manually by going to Control Panel > User Accounts. There is an option for "Change your account picture", which will provide pre-made options, or the ability to insert your own image.
You could also try replacing "C:\ProgramData\Microsoft\User Account Pictures\user.bmp" with your own 128x128 image on all computers, to replace the default image. However I have not tested this to ensure it will work in all situations.