Upload the image to a hotlinkable site, like imageshack
Copy the url of the uploaded image
In the post, add the tags [ img] [ /img] to the URL of the uploaded image (remove the space between '[' and 'img')
Be a considerate image poster. Before you upload make sure that the image is not too humongous. It is usually preferable if the resolution of the image on forum is kept within 640x480.