but doesn't.

I'm editing a drawing someone else created. It has a picture that other material is drawn on top of. The picture can't be selected, which is a good thing here.

So, one would think that this is a page background, but in Page Setup -> Page Properties there is none.

So what did the original creator do here?

The same file has other pages with pictures just pasted in and drawn over, and I'd like to set up all pages as above. But I don't see how.