I am not sure if I understand you correctly.
Normally a watermark is defined with API calls like the following:
PD4PageMark footer = new PD4PageMark();
footer.setWatermark("http://server/image.jpg", new Rectangle(10,10,200,200), 30);
(It is a bit confusing as it uses footer or header API and actually defines no footer or header)
So as you see there is no <img> tag, you just specify image URL.
The URL may also be of the following protocols "file:" - to read local image file, "java:" - to read a watermark image from JAR and "data:image/png;base64," - to pass base64 encoded images (png, jpeg or gif).