If you are using WP-Juicebox (the Juicebox plugin for WordPress) and are using the Media Library as a source of images, then you can reorder images by doing the following:
(1) Edit the page or post containing the gallery.
(2) Click the 'Add Media' button above the editor.
(3) Go to the 'Insert Media -> Media Library' section.
(4) Select 'Uploaded to this post'.
(5) Images can be dragged and dropped into a new custom order.
(6) Close the media window when done.

You can quickly and easily reverse the order of images in a Media Library-sourced gallery by doing the following:
(1) Go to the 'WP-Juicebox -> Manage Galleries' page (from the Dashboard).
(2) Edit the gallery.
(3) Change the 'Image Order' from 'Ascending' to 'Descending' (or vice versa).
(4) Click the 'Save' button.