There are a few reasons why MoI is set up to have background images as a separate thing from "regular geometry" - one big one is that it's not very good to have the images too easily selectable since they often span across a large area of the background and they could otherwise interfere with selection of geometry if they were just treated exactly the same.

Also having them in a special category made it easier to have some special modes for drawing them differently than other objects, like for example in the Image command you can set the images to draw on top of regular objects and be transparent, that lets you use them as a kind of onion skin overdrop rather than only as a backdrop for example.

- Michael