You can add new icons to FreeMind by creating the folder .freemind/icons/ and placing your custom icons there. The trouble is that if you want to send anyone your mind map, you also have to send them the icons and instructions for how to install them. It would be better if the icons used in the current mind map could be embedded in the .mm file. A 16x16 .png file is very small so it seems quite reasonable.
Is there any way to embed these icons?
note: .freemind is the folder with your property files and auto-backups (by default.) Its likely in your home directory. 16x16 .png file work; other sizes/formats might as well, but I haven't tried.