You need to add an image gallery stack for each image in the gallery. Then be sure to add the same gallery name to each image you want in the same gallery. If you want multiple galleries per page, then be sure to use different gallery names:
Also, if you want one image to show for the entire gallery, then you need to be sure only one image has the show thumbnail box checked.
Joe has a great resource where a lot of questions like this can be answered, it is his docs site:
Check it out, a great resource for all of his stacks!!