If you want to reverse the order of images in a Media Library gallery, edit your gallery (on the 'Dashboard -> WP-Juicebox -> Manage Galleries' page) and select either 'Ascending' or 'Descending' (whichever is appropriate) for the 'Image Order'.
The images are ordered by the WordPress 'menu_order' so the images should be displayed in the gallery as they are listed in the 'Insert Media -> Media Library -> Uploaded to this post' display (or in reverse order if selected in the gallery settings).
If you want to reorder images in a gallery (more than just reversing the order), then edit the page or post containing the gallery, click the 'Add Media' button, select 'Insert Media -> Media Library -> Uploaded to this post' and drag and drop the thumbnails into the order you would like them to be displayed in your gallery.