There are plenty of scripts around that will do parallax scrolling as its not something you can do without JS.
You can fix an image in place easily enough but you need js to scroll it at the same time as you scroll the page.
Here’s the first script I found but there are plenty of others.
If you want to write one yourself then you would need to adjust the background-position of a background image so that it appears to move up at a slower rate than the page is scrolling.