Which is also a link that is displayed in yellow on the side of box that you type this in when you are adding a post or should I say typing a post.
[img ]http://www.your-url-here.com/link/to/image[/img ]
To show an image on the forums to other users you must have the picture uploaded somewhere.
Linking to a picture on your harddrive will appear to work as you will be able to see it, but no-one else will be able to!
For pictures that are larger than 640*480 in size we ask that you use the [thumb] code as explained below