Unfortunately, I didn’t build this website. It’s currently set up in Joomla with the header as a module so it can easily be changed on each page. Setting it as a background image would lose some of that flexibility. Also, if the negative margin was causing the issue, wouldn’t there still be a problem?
Hm, rather than pull the slider section up, it would have been better to position the form div absolutely over it. That would work better. Yes, I didn’t look closely enough at where the negative margin was. It doesn’t matter whether you use the image in the HTML or as a bg image. (Sorry for that red herring.)