add an image to a video and keep it in the same position

The most efficient workflow depends entirely on the shot. Without seeing your shot or knowing how you rendered it we can't make a specific recommendation for a workflow. If you use Blender, you can export camera data from Blender and use it in AE. If you used C4D then you can load up the C4D file in the timeline and extract the C4D camera data. If you used Maya, there are also tools for extracting camera data. If all you have is the rendered movie then the most efficient and effective workflow may be Camera Tracking, Mocha AE Corner Pin Tracking, Mocha AE/Stabilized Corner Pin tracking, AE Motion stabilizing then adding the motion back in, or about a half dozen other tricks. No details = suggestions that may or may not work. If you are OK with simple Motion Tracking and you don't want to spend more time researching other workflows then you are done. That simple technique only works if there are no perspective changes on the surface you are trying to track.

BTW, I would give that tutorial a C- because there was very little explanation of what the motion tracker was doing and the workflow only works for one specific type of shot. The tutorial is nothing more than a very simple recipe and does not suggest any way to work around any problems or describe the type of shot the technique will work with.

The best workflow would be using external compositing data out of your 3D application. If you can't access this you could try the internal trackers. There is a lot of tutorial on using either the 3D camera tracker or just the planar one's...

OK found a way with motion tracking. Works like a charm.

https://www.youtube.com/watch?v=tqWWhChc8RA This video helped.