Hello!
I know that icon files are in .png format in the /usr/share/icons and the hidden .icons folder inside the /home/<username> folder. Different themes have different icons and icon sizes, and are broken down into several categories within the theme's folders.
The following was stolen from:
http://askubuntu.com/questions/6009/where-are-icons-stored/usr/share/icons/ typically contains pre-installed themes (shared by all users) ~/.icons/ typically contains folders with the themes installed by the user.
Also, many applications have their icons in /usr/share/pixmaps/ or in the folder with the same name as the application under /usr/share/...
As always, anyone else is free to add to any of this - or to correct me, if I'm wrong...
73 DE N4RPS
Rob