The last element is the actual image. It is defined like below:
You cannot see the watermark directly in the result view, so you need to do e.g. a PDF preview to see it.
The image is defined via a URL, that use this XPath expression:
concat('base64://',ng:base64($svgImage),'.svg')
This expression converts the SVG image into base64 (with the function, ng:base64()) and then it tells InterformNG2, that the image is defined as a base64 data string (the initial base64 text).