An editor can use the following code on a portrait image or another image that is not panoramic:
The origin corresponds to a vector from the center of the image where top left corner is -1,-1 and bottom right corner 1,1. This defines a banner focal point.
This demonstrates how an origin position can be calculated.
The client then can center the image at this point. The center point will be in the middle of the banner at all times where it can be shown without whitespace (it's worth considering what happens when an editor uses 1,0 - would it not be centered or would we show whitespace on the left?)
The focus point also allows the rendering of responsive banners that work better on smaller screens.