WebPlus X6: How to keep a background picture in same position when window is resized.

Hello again,

I have been working on my website using webplus x6.

For the website I have used an image as a background using "edit,site properties,page,background" and the image is placed at; top,centre with no repeat and does not scroll with page.
all is well until I preview and the web browser is resized this causes the image moves slightly to the left.

Here is a link to my website where you can see this happening. (please note, not on mobile, laptops or pc only)
www.gekkographics.com

Thanks in advance
Andy