Pictures are stored in the folder /var/lib/AccountsService/icons/[user name]
The [user name] is of type image/png format with a size of 96x96 pixels
Under the folder /var/lib/AccountsService/users
you have a similar file [user name] that is a .desktop format - for example
[User]
Language=en_GB
XSession=ubuntu
Icon=/var/lib/AccountsService/icons/dad
The Icon=
is not created unless you assign a picture in Users and Accounts
If you assign one of the stock pictures the file will contain - for example
[User]
Language=en_GB
XSession=ubuntu
Icon=/usr/share/pixmaps/faces/soccerball.png
Thus –as you can see– just create an Icon=
entry in /var/lib/AccountsService/user/[user name]
to point to your PNG file or overwrite the icon in the similarly named /var/lib/AccountsService/icons
folder