1for1 this watermark story comes over and over... a lot of dealers do it on any image they use for sale, personal or official, for different reasons:
1) avoid that people steal their own images
Not in this case cause an official picture was watermarked.
2) advertise their brand in case people use or link images from their website (this is the case)
Indeed, the imaged posted on this forum is linked directly from their website (and not uploaded in silverstacker forum server), and anytime a forum member displays it, their server must handle the request and consume bandwidth. I can't blame any dealer if they advertise their brand when they, actually, provide some resource to make the image public.
3) google images search engine get and display this image as search result, and in case people search for it, they know also where to buy directly from image.
This image explains the concept much better:
By the way in this image First Coin Company is doing the same thing on another coin few images ahead. Just accept it and don't care about, this is one of the most common and cheap way dealers use to spread their brand.
Alex.