Here is a table of some of our icons as of April 2013.
The name of each of these is "wpt_<name>.png".
You can include a name in the sym tag in a GPX file to get a certain icon. You dont need the "wpt_" and you don't need the ".png".
We may also interpret a wider range of waypoint symbol names, for example symbol names defined by mapsource, when reading a GPX file.