If you create a gallery and edit it at a later date, JuiceboxBuilder-Pro will reuse the source images (to create new images and thumbnails for the gallery) only if you do anything on the 'Images' tab that requires the images to be reprocessed (for example resizing them and adding or removing a watermark).
If you open an existing gallery to edit it and change only configuration options (on the 'Customize' tab), then the images in the gallery will remain untouched.

If you want JuiceboxBuilder-Pro to reprocess the images (to use new versions of your source images), a quick and easy way to do so would be to select and then immediately deselect the 'Use Watermark' checkbox on the 'Images' tab.

Please note that JuiceboxBuilder-Pro stores the location of the source images (using sourcePath attributes to the <image> tags) in the gallery's 'config.xml' file when the gallery is initially created. If the images need to be reprocessed when editing a gallery, the source images will be used only if they still exist in their original locations on your hard drive. If the source images have been moved or renamed, JuiceboxBuidler-Pro will use the images in the gallery's 'images' folder instead.

Yes (or you could just remove the sourcePath attribute completely).
However, if you do so and JuiceboxBuilder-Pro has to re-process the images, then it will have only the images in the 'images' folder to work with which may not be of as high a quality as the original source images (if they have already been resized).