What the inside of an LDS (Mormon) temple looks like is not a secret. Before the dedication of each new temple, they are opened up to the public in a several week-long open house. Many thousands of people are freely allowed inside to see the different rooms and feel the Spirit of the Lord that resides there. Pictures of the outside and inside are published by the Church. Then the building is closed to the public and is dedicated to the Lord and set apart for the work of performing gospel ordinances for the salvation of the living and the dead, the Atonement of Jesus Christ making it all possible.
See a short video inside the newest temple of the Church in Rexburg, Idaho, compiled from published photographs from the LDS Church.