@Severin
You can try to right-click, then "open image in new tab" and then try to download to disk
Most webpages, including firmware http pages "frame" those preview images with other stuff, making it impossible to access the correct path of said image.
You need to get it to a site where only the raw image is displayed and can "normally" download it from there.
I do this all the time when i want to rip single images from websites
edit: Another "awkward" way is to dig into the "source" of the page (View/Show Source code...) and try to locate the correct path, not very nice way, but you can always write an arexx script for that