Well if the image is on your computer and not on a website, you will need to upload it to a website (such as photobucket.com) From there you get the URL of the image you want to post.
While you post, there is an "Img" button near the top under the Subject line. Click that once. Type (or paste) in the URL of the image. Then click the "Img" again
You will get something that looks like
[i m g] h t t p :// URL of the image [ /i m g ]
(minus the spaces in between)
And thats how you do it.
