I can't figure out how to add new photos (and/or delete) to an existing Lightbox gallery.

Hello Randolph,

In "Windows" text menu of the Dreamweaver you will find "DMXzone Lightbox" option (last one in the list). Choosing it will bring out a new panel that allows you to manage the existing elements that are connected with the Lightbox. Standalones are those which have no group applied. Galleries you will see under a folder with the gallery's name. To add new items to the gallery you have two options:
In the DMXzone lightbox panel choose the folder named after your gallery. Click on the Settings button and DMXzone Lightbox - Gallery UI will appear. here you can add or remove items.
Second option is to create Standalone items and to add them to the gallery through the DMXzone Lightbox panel by selecting them ad choosing the galleryfrom the ggroup dropdown box in the panel.
Please, next time check out the online manual on the extension's page under the Online Help tab.

Regard,