My notes, things I have learned, and documentation I have created.

This is just a matter of knowing where the image is stored and getting on that it correct size to replace it.

The default picture, the orange flower, is stored in:

C:\ProgramData\Microsoft\User Account Pictures\

Rename the original user.bmp to something like user.bmp.orig.

Then, create or resize or find the 128×128 pixel image you want to replace it with and convert it to a bmp if it is not already. When you have the image, simply paste it in the “C:\ProgramData\Microsoft\User Account Pictures\” directory and rename it to user.bmp.