Hum,
i believe it is possible to change the shape and background colour.
Here is a Roll over icon example...
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.0">
<Document>
<description>Place your mouse over the icon to see it display a new icon!</description>
<name>Simple Rollover Icon</name>
<visibility>0</visibility>
<Style id="normalPlacemark">
<IconStyle>
<Icon>
<href>http://www.wenlink/keyhole_icon.jpg</href>
</Icon>
</IconStyle>
</Style>
<Style id="highlightPlacemark">
<IconStyle>
<Icon>
<href>http://www.wenlink/google_icon.jpg</href>
</Icon>
</IconStyle>
</Style>
<StyleMap id="exampleStyleMap">
<Pair>
<key>normal</key>
<styleUrl>#normalPlacemark</styleUrl>
</Pair>
<Pair>
<key>highlight</key>
<styleUrl>#highlightPlacemark</styleUrl>
</Pair>
</StyleMap>
<Placemark>
<name>Example Placemark</name>
<visibility>0</visibility>
<styleUrl>#exampleStyleMap</styleUrl>
<Point>
<coordinates>57.000000000000,2.1111111111111,0</coordinates>
</Point>
</Placemark>
</Document>
</kml>
<body bgcolor="#FDFFCE">
<p align="center"><FONT size="+3" color="#B37D00">Some text</FONT> <BR>
<img src="http://www.blobrana/image.jpg"></p>
<p align="center"><FONT size="+2" color="#B37D00">image text</FONT> </p>
<p align="center"><FONT size="+1" color="#B37D00">visit<a href="http://www.me.con/web.html"> http://www.me.con/web.html</a></FONT></p>
</body>