QUOTE(NightViper @ Apr 2 2009, 08:20 AM)

If it's from another website, copy the URL of the image file and click the little "Insert Image" button in the toolbar above where you're typing the post. It has a tiny image of a sunset on it (it's between "Insert Link" and "Insert Email Link"). That opens a pop-up, and you just paste the URL into the text box in the pop-up. Or type [img][/img] around the image's URL in your post.
Be sure that when you do that, you're ONLY linking to images from webspace that you personally own, such as your account on Facebook, Myspace, Imageshack, Photobucket, or Flickr-- and if you aren't a member of any of those, they're all free services that offer quite a bit of file space to upload your photos to. If you're found to be using someone else's webspace, your image will generally be removed, and you will be held responsible if the owner of said web space decides to change the image to something inappropriate.