You can achieve what you want but not with CSS, since GE Plugin lives in a different realm.
First you define the sprite as the Placemark Icon, then you must explicitly set its offset in the X and Y axis. Something like:
var icon = window.ge.createIcon('');
icon.setHref("http://opengameart.org/sites/default/files/styles/watermarked/public/last-guardian-sprites_0.png");
// Set the size of the ICON - according to the dimension on the sprite
icon.setW(50);
icon.setH(50);
// Set its offset
icon.setX(10);
icon.setY(50);
var placemark = ge.createPlacemark('');
var style = ge.createStyle(''); //create a new style
style.getIconStyle().setIcon(icon); //apply the icon to the style
placemark.setStyleSelector(style); //apply the style to the placemark
// Set the placemark's location.
var point = ge.createPoint('');
point.setLatitude(window.lat);
point.setLongitude(window.lon);
placemark.setGeometry(point);
window.ge.getFeatures().appendChild(placemark);
Also the Y axis begins in the bottom left corner of your Image.
In order to use flag-sprites you will have to re-map all the offsets (x,y), for each country flag, in your code and use those offsets when loading the placemark.
Here is the reference for the KmlIcon: https://developers.google.com/earth/documentation/reference/interface_kml_icon