a windows service is designed to run also when there are no users connected, it works like a server process always up and listening, or up and doing something, or idle.
I think what you need is a client application which runs inside every logged user’ session and eventually does the job then, if needed, communicates with the service to carry some job done.
I am saying here that instead of having the windows service running in another session than 0 you can create a small executable (probably with no UI at all) that starts up from the start up folder of all users at every user login. such application is then running inside the proper session and has access to it, it can get the screenshot then either store it somewhere itself or call some end points in your Windows Service (running always in session 0) and make the service to elaborate the screenshot taken from the client application of it.
this is the way I would do it, not trying some “magic” to tell Vista and 7 to start a service inside a session of a user that in the end is not logged in yet when the system starts.