URLs that appear within a document are formatted as links to the given URL. If the URL ends in '.jpg', '.png', or '.gif', then it is instead displayed as an image.
For example, the following:
http://google.com/ http://google.com/images/logo.gif
displays as:
You can change the displayed text to another WikiLink page by using double parentheses around the link and separating the true link and the displayed text with a vertical bar; ((WikiLink|The wiki link page)) becomes The wiki link page.
See LinkingToPoetsNames for more about practical uses for this technique.