An icon is a graphical image used within a GUI as a compact form of communication, often about an action that may be taken by selecting the icon.
For instance, an icon can be a graphical image used within a GUI application to represent some action to take. Tklib includes an ico package with specific icon-related capabilities.
The BWidget toolkit supports toolbars with icons very nicely.
You can get an excellent collection of beautiful icons, complete with the supporting software to integrate them into your Tk application, from Adrian Davis' Tk ICONS project.