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Some guides I read online talk about a folder at C:\ProgramData\Microsoft\Default Account Pictures. This directory didn’t exist on my version of Windows 8.1 enterprise so use the folder in step 4 below instead.
Create your image in paint.net as 448 x 448 save it as a BMP file and again as a PNG file, name the files user.bmp and user.png.
Resize the image to 200×200 and save as User-200.png
Resize the image to 40×40 and save as user-40.png
Copy these files to C:\ProgramData\Microsoft\User Account Pictures
If you want to manage the guest account pictures replace guest.png and guest.bmp with the 448 x 448 copies of your image named appropriately.
Delete any .dat files in the folder and restart.
The default is now this image. Handy if you want a corporate logo. The various sizes make this image visible in various places such as the top right corner (smaller image), the login screen etc.