Augmented Reality Object Tracking Formula

I was working on Augmented Reality for my Android application.

Right now, I am able to place a picture right at the position where I press with finger.

The picture will be placed right atop of camera screen and I want to make the image stay at the same world position.

For example, if a user presses a sofa, then an image will be placed right where the sofa and does not change it's position even though the camera moves.

I don't know what would be the good name for this feature, so I named it Object Tracking in the title.

I was thinking about to calculate dx, dy, and dz of the camera movement and move the image to the opposite way, but image can only move on x y coordinate, not x y z.

Is there a good formula that would make the image stays the world position?

Thank you!