A Java program that gets images from a PDF (one image per page) as Java BufferedImage objects using Qoppa’s library jPDFImages. Once images are available as BufferedImage objects, they can easily be modified before saving using Java Graphics2D. For instance here a watermark is applied to the images.

http://www.qoppa.com/pdfimages/guide/sourcesamples/PDFToBufferedImages.java

If your goal is only to add a watermark, you may also look at this simple sample:
http://www.qoppa.com/pdfimages/guide/sourcesamples/Watermark.java