如何使背景位置固定并居中?

I have my site set up in 'page-sections' for a vertical scroll. the first page-section is 100% width and height, as are the other sections, but has a background image that scrolls and stretches to the width of the browser. I am trying to maintain this as well as have it fixed position. so it holds still and the content below scrolls up and OVER the background image.I have my site set up in 'page-sections' for a