Have you tried “right click” -> “Save image as…” ?
Viewing post in Any way to download game's cover image?
Interesting, I tried it myself and it seems that’s not allowed.
As a workaround, you can do “Right click” -> “Inspect Element”, there on the selected element, you can see a variable like ‘data-gif=”<path_to_cover”’, if you open that path in a new tab in the browser, you should be able to do there “right click” -> “Save image as ..”