Use the placeholder $gallery:story_images/mygallery in the source code of the long description in a story.
"story_images" and "mygallery" are folders, which describe the path of the images. This means, you have to create the folder "mygallery" inside the folder "story_images", which exists already, and upload the images you want to display.
You can rename the folder "mygallery" but the name of the folder in the placeholder has to be the same, of course.
