Thumbnails in Juicebox and SimpleViewer galleries are all the same size as each other and, by default, are square.
You can change the dimensions of the thumbnails to match the aspect ratio of your images via the Thumb width and Thumb height options in the Thumbnails (Pro Viewers Only) section of the gallery settings page (or in the 'Site -> Customize Viewers' section for the change to apply to all galleries).
Please note that if you have images of different aspect ratios in your gallery, then because all thumbnails are the same size, some thumbnails will display cropped images.