Is it permissible to greet the Messenger of Allah, peace and blessings be upon him, and Abu Bakr and Umar, through the television during a live broadcast when the camera focuses on their graves?

Legitimacy of sending prayers and salutations upon the Messenger of Allah, peace and blessings be upon him, is legitimate at all times, especially when he is mentioned. And whoever sees a picture of his mosque or grave and sends salutations upon him, there is no harm, rather it is an act of nearness to Allah and obedience.

As for sending salutations upon Abu Bakr and Umar, may Allah be pleased with them, it is prescribed to invoke Allah's pleasure and mercy upon them at all times. Sending salutations upon them occurs when visiting their graves, not when seeing their pictures, for desirability is a legal ruling that is not established except by a legal proof.