In order to capture the screen you need to run a program in the session of an user. That is because without the user there is no way to have a desktop associated.
To solve this you can run a desktop application to take the image, this application can be invoked in the session of the active user, this can be done from a service.
The code below allows you to invoke an desktop application in such way that it run on the local user's desktop.
If you need to execute as a particular user, check the code in article