The Bulletin Board code you are referring to requires a url that points to the picture you wish to use. The syntax for all bulletin board code can be found at the following url:
http://www.indyrcsouth.org/forum/index.php?action=help;page=post#bbcrefan example of using the img tag results in the following:

the code for the above is:
[img]http://www.google.com/intl/en_ALL/images/srpr/logo1w.png[/img]
Here is another example using a photo from our photo gallery:

[img]http://www.indyrcsouth.org/gallery/albums/userpics/10001/Tex_Twin.jpg[/img]
attachments are different and require you to upload images to the attachment directory.
Look through the
help section, and if you have any other questions I will be happy to assist