The default flower user picture is displayed for new accounts and on various logon screens throughout the OS. Customizing this picture requires a simple file copy and doing so will allow you to personalize your PC or even display a corporate logo.
The default picture file is located in C:\ProgramData\Microsoft\User Account Pictures\ and is called user.bmp.
I recommend replacing the file with an image resized to 128 x 128 pixels and saveed as a 24-bit bitmap (.bmp) file for the best looking picture.
If you don’t have a good replacement you can snag one of the stock user account pictures from C:\ProgramData\Microsoft\User Account Pictures\Default Pictures.
To create a backup, rename user.bmp to user2.bmp. Then just copy your resized bitmap to C:\ProgramData\Microsoft\User Account Pictures.
I updated the article to include a sample authentication box that displays a new default image.