If you want to use for example GNOME File-Icons in Webcit, install gnome-icon-theme using your systems packaging facility. After installation do
cd /usr/local/webcit/static ln -s /usr/share/icons/gnome/24x24/mimetypes icons
You can also use every other Freedesktop.org compliant Icon theme that offers 24×24 mime Icons. You can use other resolutions, but they might not fit into the layout properly.