Twenty Eleven Theme – customised header.php, gap at the top of the page

Hi, I am using the Twenty Eleven theme and have customed the header.php file to incorporate my own banner and navigation bar.

https://kitchenideas.3dkitchensaustralia.com/open-plan-design/

In some browsers, and not even every time, my header and navigation menu are out of proportion. The ‘home’ image may be really big, or there may be a huge gap between my header images and navigation bar. If I click refresh/reload in the browser it may rectify the problem but sometimes it may not.

Where am I going wrong? Is there something I need to amend in the stylesheet?

Thanks in advance for any help provided.