Well, first you have to upload your pic. You can go to your gallery by clicking the 'gallery' link at the upper left of the screen and then clicking 'upload' on the link panel across the top of that screen.

Just upload your pic, then paste the image address (you can get this by right clicking the image and 'copy image address') in the img tags [ img ] image address goes here [ /img ] -just without the spaces between the [ ]'s