FR's board code follows html 4.0 standards pretty closely. A few things it won't let you do, but nothing all that interesting.
Basically, just type <img src ="http://yourpicutrelink.com">
Be sure to preview to make sure that your source isn't blocked/redirected from the link by the photo hosts server. Also, can can use WIDTH= and HEIGHT= attributes inbetween the close quote and the > to help up or down size a photo to fit a bit better.
Examples...
Full sized photo.
Width set to 250 pixels.