I was looking to place an image behind Struc, land, particles... everything, really. However, not ImageLayer ("TheBack"), ImageOrder (-1), ImageOffsetZ (-100 before abruptly becoming invisible), or any combination of these worked.
This is a pity, because I was looking to bury a structure, without inviting lag by using a whole crop of pixel-perfect images.
I'll use a transparent overlay, but is there a better solution?