Hi Marty,
The background image is using a script called “backstretch” – the result is that your background image will always stretch to fill the available space. There’s not a way to avoid it, since that’s the nature of it.
Your best options are to 1) use a photograph that will look good whether a lot or a little is showing, 2) ditch the background image altogether and use a widget in that space to insert your image, which will then always be confined to that area and look better at different zoom levels.
You could re-purpose the existing Welcome text widget area to do this, but it’d take some advanced CSS edits.
Cheers,
Carrie