It looks to me like the background images aren't actually background images...the site has the background images and the quotes in sibling divs with the children of the div containing the images having been assigned position: fixed; The quotes div is also given a transparent background.
wrapper div{
image wrapper div{
div for individual image{ <--- Fixed position
image <--- relative position
}
}
quote wrapper div{
div for individual quote{
quote
}
}
}