Once the image is on the web, to link the picture do this:
pjc30943 wrote:...If you press the "Img" button above the reply box, you'll see something that looks like "[img ][ \img]". Put your link to the picture between the two brackets such as [img ] <link here> [ \img], and that photo should appear on the preview page when you get to it...
More info: http://www.summitpost.org/faq