Is Juicebox plugin still being supported?
Yes. WP-Juicebox is still supported. However, as far as I am aware, there are no immediate plans to change the plugin's current functionality.
If you choose the Media Library as the source of images for a WP-Juicebox gallery, then the plugin will display all images attached to the page or post containing the gallery. You can arrange the images in the media window's 'Insert Media -> Media Library -> Uploaded to this post' section (where you can drag and drop the images into a custom order). WP-Juicebox displays the images in WordPress's ascending menu-order. Initially, images will be displayed in the order in which they finishing uploading but once you drag and drop any images into a new order, this is the order in which they will be displayed in the gallery.
WP-Juicebox displays the image's 'Title' and 'Caption' (entered in the media window's 'Attachment Details' pane) in the gallery's caption area.
Instead of using the Media Library, you could use a Flickr account or a NextGEN gallery as a source of images for a gallery.
As an alternative to using the plugin, you could create a Juicebox gallery on your computer (perhaps using JuiceboxBuilder), upload the complete gallery folder to your web server and embed it into a WordPress page or post using the baseUrl method of embedding documented here. Be sure to paste the embedding code into your WordPress page or post in 'Text' rather than 'Visual' input mode. It does not matter where on your web server you upload your gallery folder to as long as the two paths in the embedding code (the path to the 'juicebox.js' file and the baseUrl itself, pointing towards the gallery folder) are correct.