You need to use BBCode. For example, to link this text to this topic, I would do this:
CODE
[url=http://runescape.salmoneus.net/forums/index.php?showtopic=294243&hl=](insert text here)[/url]
if you copy+paste what's inside the code box, you'll end up with a link to this topic.
Like this:
(insert text here)for a pic, you'd do the same thing, except you use the code for a pic in place of "(insert text here)"