With the help of SMF forum member Arantor I was able to add a custom button to my Simple Machines Forum 2.0 (Warez-DnB)
To add a custom button you need to make an edit in the Sources/Subs-Editor.php file.
Locate
// The below array makes it dead easy to add images to this control. Add it to the array and everything else is done for you!
$context['bbc_tags'] = array();
$context['bbc_tags'][] = array(
You can now see how a button is made up within SMF 2.0 for this example we will use the [img] button. 
'img' => array('code' => 'img', 'before' => '[img]', 'after' => '[/img]', 'description' => $txt['image']),
You can see how this is made up, to add your own function (in the case of Warez-DnB a button that will atomically copy in the text [img]http://images.my-domain.com/nocd.png[/img]
I added
'noimg' => array('code' => 'noimg', 'before' => '[img]http://images.warez-dnb.com/nocd.png', 'after' => '[/img]', 'description' => $txt['No picture']),
You’ll also want to add the button within your theme’s images/bbc folder that needs to be the same file name as the function, in this case “noimg.gif” 
Simple stuff but handy to know.