Host the image with a photo site such as Photobucket.com or Thumbsnap, then copy the url for the image (Photobucket has a facility which even adds the image tags from the thumbnail).
Then in your post, click the IMG button above, then paste in the url for the image.
If you use Photobucket, you don't need the IMG button, just paste the link they provide.
Any other images from the web need only right click to copy the image URL, then paste the same way.
Thus [
[/IMG]http://i15.photobucket.com/albums/a399/Engraver/Dinosaursmissboat.jpg[/IMG ]
becomes
when the spaces and dots are left out!