I don't know how to add the pictures directly.
Right-click the image, copy the URL (with WikiPedia, they append a "File:" description for the entire image page with attribution, et al).
Then, use [img ]http://www.site.com/file.jpg[/img ]
(remove the space after "g" but before the "]".)
Quote an image in someone's post, and edit to see how it works.