How To Find Icons in Linux
It's really low tech having to browse subfolder upon subfolder in those locations looking for the right icon.
But, now that we know where are most icons, we can help ourselves a lot by configuring the Tracker search demon to watch those folder along with our home folder. Tracker is the search engine which Gnome uses to index files in folders and watch those folders for changes in periodically. The configuration dialogs are accesible from System > Preferences > Search and Indexing.
Once we set up Tracker to index and watch those folders and enable the option to geerate thumbnails (if we also isntalled the tracker-tools package a pop up will tell us when the tracker daemon has finished indexing) the faster way to launch Tracker Search Tool is via GnomeDo: calling it via the shortcut WindowsKey+Space and typing Tracker then Enter --GnomeDo will later guess that if we type just "tr" we probably want to launch Tracker again.
There, in the Tracker search text box, we can benefit from the subdirectory structure used in /usr/share/icons/ and type something like "*.png 24x24" which will return just a list of png icons that size.