When you place an image in a schematic, it seems to be drawn in front of any previously-placed object that it overlaps; similarly, any object placed on the schematic afterward is drawn in front of the image.
In my application, I am using an image as a background guide to where components and wires are placed, and consequently need the image to always be drawn behind all other objects in order to avoid obscuring the very objects I am trying to reposition. (The workflow, due to Bug #1785353, makes it impossible to insure that the image has always been placed *before* all of the objects whose placement it is guiding.)
There is a workaround for this, in that it's possible to do a block cut on all objects in an area, then place an image there, and then paste the block back, but this is a real hassle.
It would be better if there was simply a setting to say that an image had to always be drawn first (or at the lowest Z-level if there were such a thing).
And actually, the workaround I mentioned wouldn't work anyway, since doing a block paste like that resets all of the reference designators. I haven't actually been able to think of any other workarounds, other than editing the .sch file directly in a text editor.