You can use your own graphic icon with any of your placemarks in Google Earth. To do this, create or edit a placemark. In the New/Edit dialog box, click the placemark button in the top right corner. The Icon dialog box appears. Click an icon or to add a new icon, click Add Custom Icon.
Note that you can also set the opacity and scale of the image. Of course, you'll want to optimize the file size of this image so as to keep Google Earth performing well.
You can set the icon in the KML source as well. Learn more.