icon-theme provides a facility for looking up icons by name and size. The main reason for using a name rather than simply providing a filename is to allow different icons to be used depending on what icon theme is selecetd by the user. The operation of icon themes on Linux and Unix follows the Icon Theme Specification. There is a default icon theme, named hicolor where applications should install their icons, but more additional application themes can be installed as operating system vendors and users choose.
Named icons are similar to the Themeable Stock Images(3) facility, and the distinction between the two may be a bit confusing. A few things to keep in mind:
Stock images usually are used in conjunction with Stock Items(3)., such as GTK_STOCK_OK or GTK_STOCK_OPEN. Named icons are easier to set up and therefore are more useful for new icons that an application wants to add, such as application icons or window icons.
Stock images can only be loaded at the symbolic sizes defined by the GtkIconSize enumeration, or by custom sizes defined by gtk_icon_size_register(), while named icons are more flexible and any pixel size can be specified.
Because stock images are closely tied to stock items, and thus to actions in the user interface, stock images may come in multiple variants for different widget states or writing directions.
A good rule of thumb is that if there is a stock image for what you want to use, use it, otherwise use a named icon. It turns out that internally stock images are generally defined in terms of one or more named icons. (An example of the more than one case is icons that depend on writing direction; GTK_STOCK_GO_FORWARD uses the two themed icons "gtk-stock-go-forward-ltr" and "gtk-stock-go-forward-rtl".)
In many cases, named themes are used indirectly, via GtkImage or stock items, rather than directly, but looking up icons directly is also simple. The GtkIconTheme object acts as a database of all the icons in the current theme. You can create new GtkIconTheme objects, but its much more efficient to use the standard icon theme for the GdkScreen so that the icon information is shared with other people looking up icons.