Gallery creation guidelines
In order to avoid various quirks and glitches with your galleries please follow these instructions when creating a gallery:
Go to galleries -> new gallery item and give a title to your gallery.
Then hit the "Add media" button and in the popup media manager, upload your photos and when they are done uploading reorder if you wish and press the "X" button located on the top left to close the media manager.
--- To change the image order drag the images in the order you wish to view them just before hitting the X button.
(please notice: do not select "create gallery" from the leftmost list, do not use images that already are in WordPress's media gallery and in general if you want to use an image already uploaded in a different post upload it again to the new one, do not use the one already uploaded).
Edit the "gallery settings" by adding venue and location (optional).
Click the 'Set featured image" link located on the right side of the page, set a featured image. Add any gallery categories (if needed).
Publish your gallery.
If you want to add new images to an existing gallery, just upload the new images just like you did the first time when creating the gallery, they will be automatically added to the existing ones.
If you want to reorder or delete images after the gallery has been created, edit the gallery item, hit "add media" from the dropdown menu on the left side of the media manager select "uploaded to this post" and reorder with drag & drop (or remove) any images you want, hit "X" and update the gallery.