Say I found this picture and I want to post it:
You go on the site that you found it, right click on the image (or Ctrl+click on a mac), copy the image URL to clipboard, which looks like this: http://t2.gstatic.com/images?q=tbn:ANd9GcR9Xy8YK4TnaKSyPVWyH1vrtVnoXn45UTmSO0Sy9SEH__Kmxfkd
You paste that URL into the message window, highlight it, and then click the img box. The tags [ img] [/img ] will surround the link (w/o the spaces, though) and automatically get converted to the HTML tags you need to link the photo. When you click "submit," the image will be linked to the post.