Sarah,
Some images are able to be linked to directly from where they reside in the Internet. You then put the link between the
[img][/img] code tags, and hit the "Preview" button to see if it worked. I'm not exactly sure why sometimes it works, sometimes it doesn't ...
If all you see is text, you probably need to save the image to your hard drive (right click on the image and save), then use one of the image hosting services the guys provided links to above. Then post the link to the image there in the same manner as described above.