In the google_javascript/wagmp_map_[mapnum].php file, you would want to find the code that looks like this:
var icon_0 = new GIcon();
icon_0.image = 'plugins/google_javascript/images/traditionalflat_pacifica.png';
icon_0.shadow = 'google_javascript/images/traditionalflat_shadow.png';
icon_0.iconSize = new GSize(34,35);
icon_0.shadowSize = new GSize(34,35);
icon_0.iconAnchor = new GPoint(9,33);
icon_0.infoWindowAnchor = new GPoint(19,0);
icon_0.printImage = 'plugins/google_javascript/images/traditionalflat_pacifica.gif';
icon_0.mozPrintImage = 'plugins/google_javascript/images/traditionalflat_pacifica_mozprint.png';
icon_0.printShadow = 'google_javascript/images/traditionalflat_shadow.gif';
icon_0.transparent = 'google_javascript/images/traditionalflat_pacifica_transparent.png';
For each of the points, there will be another block of code that looks like that but with a different icon_[num]. You would want to change the icon here and make sure you have the images stored in the google_javascript/images directory.