When you use a watermark in JuiceboxBuilder-Pro, the watermark is baked into the image itself when the image is resized for the gallery. The idea is that wherever the image goes, the watermark goes, too. It's usually used as a deterrent to discourage people from downloading your images.
As such, the watermark itself cannot be moved from the image to somewhere else in the gallery.

However, you can add an image (such as a watermark image) to your gallery in several other ways.
Please see the Adding a Logo support section for a couple of suggestions.
Perhaps the best suggestion would be to use the Gallery Title (suggestion #3 in the link above). If you set galleryTitlePosition="TOP", then you can also set topAreaHeight to a value large enough to ensure that the image you use does not overlap the main images in your gallery.

If you just wanted to include some copyright text, then you could either use the Gallery Title or the image titles and captions.
Image titles and captions can be multi-line so if you already have a caption for an image, you can add a couple of HTML line breaks and then your copyright text to the end of an existing title. For example, you could enter a caption such as:

This is the image caption.<br><br>Copyright © 2017 Me