When customizing linux desktops you will probably want the icons to match the applications.
Most application designers include icons - but where?
One way is to simply try and find it in your system (e.g. mozilla firefox) find / -name 'mozi*.png'
The "default location" (if there is such a thing in linux) is one of the following places:
/usr/share/icons /usr/share/pixmaps /usr/share/applicationname/resources
You'll most likely have to browse through a number of folders (e.g. apps or kde) but they're there!