First and most importantly, be sure that the photo is stored somewhere on the internet.
Second (two steps):
1) Simply type out the full URL to that image (like http://www.yoursite.com/blah/image.jpg
2) On this board be sure
to surround that full URL with IMG tags like so... [ IMG ]the URL here[/ IMG ] but remove the spaces from my IMG tags. You can use the "IMG" button to do this IMG thing automatically.
Hope this helps!