Vista - Force user account picture w/Group Policy

(Previously posted to microsoft.public.windows.group_policy, but it
looks like the answer isn't available as a group policy, so I am more
looking for a registry key or way to script this, which I will then
deploy via GP)

Does anyone know if it's possible to use a Group Policy (or GP
preference) to set user account pictures?

The thought would be to copy user pictures out from a central location
on the LAN to a predefined location on each PC, and then use a registry
setting (or whatever method is appropriate) to force the appropriate
image to be used as local pictures.

We're running a mixed Vista and XP environment, although it's really
only Vista machines that we're concerned with since XP doesn't make as
big a deal over the user pictures.