You'd first have to upload it on to the web (if you don't have your own site, try railroadforums.com, railpictures.net, or the like) of course.
Then right above where you put your password below the text field it says "Optional Image URL". Just go view your photo wherever you posted it and put the photo filename (right click, "properties", copy and paste the URL to it...or just link the page).
Hope this helps. Note RR forums won't usually allow you to link just the image without the whole little image-view page.
-Ed Kelley