In fact graphic smilies are just small pictures, if they are associated with some special codes in the forum (":)" or ":(" for example) they get shown in posts by typing those codes. But if they don't have those codes, they still can be inserted same way as images, i.e. using the [img] tag.
Steps to post external smilies:
First, you need a hyper-link for your smiley, if it's already on the Internet (on another forum for example), it has already a hyper-link that you could copy to later put in your post; if the image (smiley) is on your computer, upload it to an images host to get a link for it. We already saw the two methods in detail when we talked about images. See this: Methods to Get Images Links.
Now that you have the URL (hyper-link) for your smiley, you just have to wrap the link with the [img] tag & you're done (for more details see: Intro on [img] tag & Hyper-links).
Example:
I uploaded a smiley to PhotoBucket, and here's its hyper-link:
http://i228.photobucket.com/albums/ee88/learn-bbcode/smile.gif
Now let's wrap it with the [img] tag:
[img]http://i228.photobucket.com/albums/ee88/learn-bbcode/smile.gif[/img]
Once you have that code you can insert it anywhere in your message, it's much longer than usual smiles codes but the result is the same:
Hi! [img]http://i228.photobucket.com/albums/ee88/learn-bbcode/smile.gif[/img] Howdy?
Would look:
Hi!
has the code