Page Images

I am using the Aegaeus theme and don’t have much experience with WP. I was in the appearance settings and saw the default image and I changed it. I am aware that only boxed-layout allows the images for the pages to be used. But, when I scroll down the image background moves too and then it shows a gray background below as I am scrolling. How do I make the page remain static or the images so the images are not scrolling with the web page. I am not sure how to fix this. Please advise when you get a chance, thanks.

Robert