If you have a Dropbox account, created before October 4th, 2012 (meaning you have the public link feature) you can follow this tutorial:
1/ Install the Dropbox app
2/ Download some icons. Right-click on an icon and “Save image to” your hard-drive.
3/ Place the icon files in your Dropbox “public” folder. You may create a sub-folder “mapicons” and place all the icons into it.
(Dropbox tutorial : How do I add files to my Dropbox?)
4/ Get the Dropbox URL of each icon : right-click on a Dropbox file and select “Dropbox > Copy Public Link”.
(Dropbox tutorial : How do I link to files in my Dropbox?)
5/Then in Google “My Maps”, edit the marker, and choose “my icons”, “add icon” and paste the url of the icon previously copied in Dropbox.