Anything which is selected is visually on the top, as long as it is in this state. I don't know a way to alter this.
It's true, if an image is moved or scaled, it seems to be disappeared, and brought back by AutoZoom. This has been fixed in the latest 2.2 RC version.
It works for me to change the order of several images. It works best to activate the action first, then select the object(s), close the selection with Enter, then an Escape to get the objects out of selected state.
There is one real problem: if you have some line objects in the image area, and you want to select them for some editing, always the image will be selected instead, even if it is on an extra and locked layer. You have to switch that layer to invisible to prevent this. So you can use an image as an underlay only during drawing, but not when you want change something. Or, if possible, use only a selection window, to select only the line and not the image.