It's possible to change the default icon. Best to use is a .png file. A .ico bestand isn't rendered with IE7!

{mosmap icon='<Url icon>'|iconwidth='60'|iconheight='52'|iconanchorx='58'|
iconanchory='52'|iconinfoanchorx='34'|iconinfoanchory='1'|iconshadow='<Url shadow'|
iconshadowwidth='60'|iconshadowheight='52'|iconshadowanchorx='34'|
iconshadowanchory='0'|icontransparent='<Url Trans>'|iconimagemap='<image map>'|tooltip='Tooltip'
|text='Custom Icon'}

Even an animated icon (.gif) works:

For a icon wizard goto Google Map Custom Icon Maker.