Thank you for the feedback about the form fields, we are happy to hear that you like the functionality.
To change an image in a document, you can use the ImageSource property of the Image class and set the source to point to the new image. More information on how you can work with the source is available in the ImageSource topic.
As to the watermark, in the PDF document format, there is no a concept for watermark but the flexible manner of drawing the objects allows you to draw it on a page. I am attaching a sample project showing how you can use the API to add a text watermark. The method containing the implementation is called AddWaterMarkText().
When working with already generated documents, another option is to use the PdfStreamWriter API. Here is an example of how to add a watermark to a PDF document using this approach.