First of all, make sure that you select 'Uploaded to this post' in the WordPress Media Window to be sure that the images you are reordering are attached to the post containing the gallery.
Next, clear your browser's cache to be sure that your browser is fetching and using the most recent gallery files from your web server.
I am trying to add new photos to a gallery
This can be done with the following procedure:
(1) Edit the post containing the gallery.
(2) Click the 'Add Media' button above the editor's toolbar.
(3) Drag and drop your new images from a folder on your computer into the 'Add Media' window in your browser.
(4) Close the 'Add Media' window with the cross at the top-right.
Alternatively, if you already have some images in your Media Library that are unattached to posts, you can go into your Media Library (from the Dashboard) and attach them to the post containing the gallery.
deselecting those already uploaded doesn't work
Selecting or deselecting images in the 'Add Media' window does not add or remove the images from the gallery, it just prepares a selection for a bulk action within the 'Add Media' window (such as 'Insert into post.
Do I have to delete images associated to the page?
WP-SimpleViewer will display all images attached to a post so if you want to remove an image from a gallery, you will need to either unattach it from the post or delete it from the Media Library completely.
There is no images XML file to edit.
There is no XML file as WP-SimpleViewer generates the image data when the gallery is displayed. This allows you to make changes in the Media Library without the need to rebuild a static XML file afterwards.
If you want to check a gallery's dynamically-generated XML file, it will be in the form:
Just enter the URL into your browser's address bar (changing the domain and Gallery Id as appropriate).
I hope this helps clarify matters.
SimpleViewer Support Team.