Keep a copy of the gallery on your computer and you can edit it using JuiceboxBuilder-Pro.
You can add or remove images on the 'Images' tab and then re-upload the gallery to your web server when finished.
If you make no other changes to the gallery than adding or removing images, then you need only re-upload the gallery's 'config.xml' file and the new images and thumbnails.
If you simply upload your entire gallery to your web server, most FTP programs should be smart enough to upload only new or changed files (though you may want to manually delete images and thumbnails that are no longer included in your gallery).