This is what I do. I go into the gallery, select the photo in whatever gallery it's in, then right-click and select "Open image in new window". This will open a window with just the picture. Then I place the link in the post, like this:
http://www.yorkietalk.com/gallery/da...89100_2378.jpg
I'm sure there's an easier way. If I want the thumbnail on my thread, I just go to "manage attachments" and take the photo directly from my hard drive.