If you are using a WindowManager instead of a DesktopEnvironment and you have no icons for folders and files in pcmanfm or thunar or other similar applications, you have to specify a GTK+ icon theme.
Create ~/.gtkrc-2.0 or /etc/gtk-2.0/gtkrc and add the following line:
gtk-icon-theme-name = "icon theme"
where “icon theme” is some of your installed icon themes e.g. “nuvola” or “oxygen”.