This is not XWiki specific, but a CSS feature. In your stated example, the CSS code is: <style> .wikiexternallink { background: url("[...]/externallinkicon.gif") no-repeat scroll right center transparent; padding-right: 12px; } </style> C.f. http://en.wikipedia.org/wiki/Cascading_Style_Sheets for more information about CSS.

i want to mark external links in my wiki like it's done here: http://www.xwiki.org/xwiki/bin/view/Main/ExternalLinks with a symbol after the link. How can i do that?