By turning auto-fill off, you maintain the dimensions of the image you've uploaded, rather than adapting to the frame, and you can use the alignment settings or hold down [alt] to decide where the image sits within image frame, horizontally and vertically.
This opens up a world of flexibility, allowing you to layer your images with feature boxes or other images in interesting ways that aren't bound by grid structure and the number of columns you've chosen for the immerse section: