No problem. It's really not that difficult.
Like Joseph already said, just go to your gallery, click the thumbnail of the picture you want to add and under it you will see for instance:
UBBCode: [img*]http://gallery.viperclub.org/data/500/6041DSC00603-med.JPG[/img*]
(without the two asterisks)
Just paste the UBB code where you would normally type your message. Finally, the image should be added to your post.
Good luck!
