Once you can see the photo online and load it up to view it at that site, you can get the actual link to the photo by right clicking on the photo and selecting "properties" and then highlighting the properties location of that photo and copying that URL by hitting CTL+C on the keyboard. Then you can display it here by putting the image command before and after the link to the photo which looks just like this:
Make sure the link you highlight, copy and then paste has a JPG, or GIF, or whatever the photo was at the end.
www.pbase.com
www.photobucket.com
are both free and can host up to a certain amount of photos that you can then post to this forum.
You can also reference this link for a description of how to create posts:
http://www.phpbb.com/phpBB/faq.php?mode=bbcode#7