This is one way to create a library of common icons, accessible through user-defined TWikiVariables set in WebPreferences, or in TWikiPreferences for site-wide use.
There are other approaches as well:
In any topic, write %PUBURL%/%TWIKIWEB%/TWikiDocGraphics/help.gif to show the icon
The TWiki:Plugins/SmiliesPlugin, using a different short hand than %VARIABLES%, may be worth checking out for extended use
Related: TWiki's FileAttachments show their own icons for many different file types. As documented in TWikiVariables, write %ICON{"pdf"}% to show the icon
In this example, TWikiDocGraphics - this topic - acts as the icon library. Graphics are uploaded and displayed. Then, the paths are set in TWiki.TWikiPreferences#DocGraphics for site-wide access. You can also create individual library pages in each web. And you don't have to restrict yourself to graphics: Preferences Variables can include text and links to any type of file.