You can add a photo in by link using the photo link button in the tool bar above the edit area (little photo icon 2nd from left), in which case you put your address between the tags that button inserts, or you can do it by hand almost as easily as well with code like:
<img src="_the_link_address_with_http_goes_here">
Just type that in and then use preview button to verfiy that the image is showing up. If it's not then you have mistyped the address or something in the html code. What you're doing is using the IMG tag and setting the source address. Other fancy things can be done but for that read up on HTML.
The benefit to doing this is that it does save server load, space and bandwidth for the forum and means you don't have to upload the image again. The downside is that if you remove your photos from that other place they will also show up "dead" in the forum here.
Chris :)