When you post (on almost any forum), there's a little square with a mountain in it up there ^^^ while you're typing. (Hint: if you hover your pointer over it, it says "insert picture".) Click that, then put the url of the image into the pop up, click ok. Your picture will appear.
If there is no little mountain picture thingy, type this [img]picture url goes here[/img]
Ninja'd less sarcastically by Ralphy.