Yes. Easily. Go to C:\Program Files\Google\Google Earth\res\shapes
and change those. They correspond to your placemark icons.
Edit: Let me be more precise. That folder allows you to edit the bottom half of the icons (in attachment, the ones with inverted colors).
The top ones can be edited in this folder:
C:\Program Files\Google\Google Earth\res\paddle
Also see attachment. They are now also inverted.
And finally, the last ones remaining (the standard pushpins) are found in this folder:
C:\Program Files\Google\Google Earth\res\pushpin
Also, make sure when editing the images you keep them as .png and you don't change their canvas size.