I"m very confused. Do you want to insert a photograph of the sun in a video shot with a moving camera??? If so then there needs to be something in the video that you can track. Then you apply the tracking info to the 'sun'. Another option would be to use Camera Tracking to analyze the camera movement then find a target as far away from the camera as possible and add a null and a camera. Make your sun a 3D layer (it may disappear) then hold down the shift key and make the null the parent of the sun. The sun will snap into position. Now just drag it into the spot in the sky where you want the sun to be.
Another option is to stabilize the shot so that it is not moving, then add in your sun, then re-introduce the motion to the shot. Here's a quick tutorial that shows how to do this. The target would be the sun layer. You would not set up a track matte, but you would just find something in the movie that you could track to stabilize the movie so that it does not move. If there is a lot of camera motion then you would need to use two points. Then you would add your sun and follow the steps to add movement back to the movie and apply the inverse of the motion to the sun.
If none of these techniques work for you please give us a better description of what you are trying to do.