It looks like there’s a transparent div on top of the img element, so when you think you’re right-clicking on the image, you’re really right-clicking the transparent div. Kind of annoying. This isn’t a behavior I would recommend imitating.
I’m not even sure they were doing this to purposefully block people from viewing the image. It might have just been an accident. MANY years ago this sort of thing might have appealed to people who thought this would keep people from stealing their images, but that’s a very outdated way of thinking (anyone competent will be able to get to the image anyway), so I doubt that’s it. And if it is the case, that’s rather lame.
Pro tip: I see you’re using Firefox, so if you want to access any image on a page, you can right click anywhere on the page and choose “View Page Info” or “View Image Info”, and make sure you’re on the “media” tab of the window that pops up…then just keep scrolling through the images until you find the one you want.