I would center it on the door.

it would look odd to be centered on the ceiling of the porch especially if it is a drop light. if it is a flushmount, it might not look as bad if you opted to center it on the ceiling. however, i would still center it above door.

depending on the size of the light and your need for light in the area at night. you might consider a drop type light centered over the door and a flush mount light elsewhere on the ceiling centered or not.

I agree with Kelley. Not only will it light the entryway better centered over the door, but it will be lined up with the porch when you walk up the stairs. It wouldn't make a huge difference either way, but I would recommend centering over the door.

What about recessed lighting as ceiling looks low and then you would not have to worry where to centre it so much.