Save the attachment to a directory in your computer. In my Linux system, Google Earth appears to use $HOME/.local/bin/google-earth/resources/shapes. I also save a copy in $HOME/.local/share/icons, where I keep various icons for my own use. In my Windows system I save custom icons in C:\Wbin\Icons.
When you "add a place-marker" in Google Earth, in the screen where you select the icon, pick "custom icon." Then go to the directory where you save the new icon and pick the "winnie.png" icon, or whatever name you assigned to it.
This is for version 5.2, the one I use. I'm not sure if the newer versions will let you use a custom icon. I've read that the new versions have the icons in a single file, so you'll have to save it to your own directory.
2006 Sunseeker 3100SS - "Heart Of Gold"