Folks, I've spent a while troubleshooting this, so I thought I'd share the knowledge... I had inserted images into a post, then embedded a Slideshow into the site Home Page using [slideshow post_id=210].
Works like a charm! Until... I removed a few images in the post and rearranged the remaining images in the post and resaved it. No changes in the Slideshow Gallery -- it still showed all the original images and in their original order. Furthermore, when I added an image to the post from the WP Media Library (in other words, it was previously uploaded for use in another post), it did not show up in the Slideshow at all.
Here's what I've learned: the Slideshow plugin does not pull the current images that are being displayed in the post; it pulls the images that are associated with that post's Gallery (hence the name "Gallery Slideshow," if I'd been thinking about it at the time...). So, editing the Gallery (Click "Add an Image," then in the popup click the Gallery tab) also edits the Slideshow. In that gallery editor, you can change display order, reorder, or delete images. On the other hand, if you've inserted images from the Media Library, as I did when I went back and edited the post, it'll have no effect on the Slideshow.
So I dug a little deeper. A Gallery is created for images when you upload them to a post... and that gallery is only for that post. As someone said, the uploaded image is a child of the post. Meanwhile, the Media Library helps you reuse your images, but it doesn't change those linked relationships. So if you have images that are in your Media Library, you are able to display them in a post, but that does not make them a part of that post's Gallery.
Weird? Yes. (But remember, you're getting all this programming for free.)
As far as I can tell, there's no elegant way to associate an image from the Media Library with another post so that it can be included in the Gallery for that post. You can upload the image again (creating dupes on your site), or delete it from the Media Library, then upload it from within the post with your Gallery, then update any other links to that image (provided they are not part of a Gallery for the other post). Not great solutions, but they're all I could find.