It would be the same as posting a pic from a computer. First you need to get your picture on Photobucket. THen you need to know what the URL is to the image on Photobucket (they usually provide links). If they have a set of links that include the tag, you can use those directly. If you just have a URL that starts with "http", then when you post here, wrap that URL in IMG tags like so:
[img]http://somelinkonphotobucket.com/to/my/image/473647335.jpg
-Alex