See 
PublishedAPI for packages intended to be used by Plugin and Contrib authors, or 
browse all packages.
See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
 StaticMethod render($session, $url [, $alt]) → $html 
Generate the output for representing an 16x16 icon image. The source of
the image is taken from 
$url. The optional 
$alt specifies an alt string.
re-written using TMPL:DEF{icon:image} in Foswiki.tmpl
%TMPL:DEF{"icon:image"}%

%TMPL:END%
see 
SkinTemplates:base.css for the default of .foswikiIcon img