All of the pictures are stored in a hidden folder buried deep within your program data directory.
C:\ProgramData\Microsoft\User Account Pictures\Default Pictures.
All you have to do is just remove the pictures you don’t want, and add the pictures you do want. You just need to make sure that your pictures are 128×128 resolution in bitmap format. If you want the pictures to be ordered in a certain way, you can name them in order.
If you’d like to copy your current picture, it is stored in the following file:
C:\Users\<username>\AppData\Local\Temp\<username>.tmp.bmp
The default user picture (the flower) is stored in the following file:
C:\ProgramData\Microsoft\User Account Pictures\user.bmp