Unfortunately, changing the zoom level on image maps is a little finicky do to the way they are built in the module I am using (LeafletJS) to create the maps. I try to detect this and reset marker positions, but it might not always work (seems like it doesn’t in your case). Your best bet here is to set the image bounds (see this discussion for an example).
Also, zoom levels in Leaflet are not intuitively named. Max zoom is how far you can zoom in.