CSS Scaling image with browser zoom

Hi
I'm trying to build a wordpress site. I'm relatively inexperienced in CSS and during testing our administrator pointed out something and I'm trying to see if there is a solution or not.

We have a fairly plain background made of a repeating image. I wanted to put our church logo on top of that background - which I did with the following CSS:



The problem seems to be that when someone uses the zoom function on their browser (which the administrator does as he uses a tiny netbook and has failing eyesight!) - the cross stays full size and destroys the layout of the page by getting in the way of the text and other areas of the page.

I'm a little bit loathe to put the URL here as it's running off a no-ip address as it's currently stored on our office NAS while I build it, but if people need it I will.

Is there anyway of preventing this? Or is it just something we have to put up with?

Thanks

Nick