Image displays correctly on Kindle Fire but not Kindle Paperwhite

[Cepheus]

Hi, we have a mobi book with a background image that is supposed to display at the top of every part and chapter opener. It is working fine on a Kindle Fire but on the Paperwhite it displays on every page not just the chapter opener. Has anyone else run into this problem and how was it solved. We are thinking maybe we need to use a media query but are wondering if other people are having similar problems. The code we are using is below:

/*img*/
section.part, div.part {
background-image: url(../images/e_sun.jpg);
background-position: top center;
background-size: 18% auto;
background-repeat: no-repeat;
}

section.chapter {
background-image: url(../images/e_sun.jpg);
background-position: top center;
background-size: 15% auto;
background-repeat: no-repeat;
}