From the gallery here you can simply go to the picture you want to embed, and then copy the BBC embed code underneath the picture (it looks like [ smg id= number ] ). Paste that in your message in the forum, and it will show as a thumbnail in your post. People can click on it to go to the bigger image, and even check the info that was posted along with it.
For external images please make sure hotlinking is allowed for that server, and personally I would say; only link images that are
yours. You can use the [ img ] tags, I think are also shown in the elaborate reply settings (don't use Quick reply, it shows no formatting options).
[Edit] When you click Modify on your post, you can also get there, like I'm doing now.
The image linking icon is the one underneath the Bold text setting. Hoover over it and it says Insert image. Paste the URL between the img codes.