I am new boy on the block to inkscape using ubuntu 14.04. I have photographs of artwork I wish to protect with watermarks but cannot navigate my way around inkscape sufficiently well to add a watermark to my photos. I would be grateful if anybody can give me some useful advise.

In my opinion, Inkscape is not the first tool for this job.

Try a raster program instead, like Gimp. You can get good support on gimpchat.com.