Terry, the URL for the picture has to be inside image tags like this:
[img]https://farm8.staticflickr.com/7442/12679461185_fc863136a5_m.jpg[/img]
Then they will display:
Make sure the link is to the picture and not to an album. If the photo hosting site doesn't give you an option that includes image tags, then the easiest way is to copy the picture's URL, then in your post click on the "Insert Image" button (right below the "Bold" button), this will insert a set of image tags, then you can paste the picture link inside of them.