Okay. The trick is that Discogs will only show you a 600x600 image even if they have a higher res version. With your Dutch example above this makes no difference as they only have 600x600 images stored.
With other albums (https://www.discogs.com/release/15771199-Peace) you can get MUCH higher resolution images out of Discogs. You just need a tool like this to manipulate the URLs for you.
Lets use the my copy of the Levellers - Peace I uploaded big images for a few months back.
Go to EDIT RELEASE, then EDIT IMAGES: https://www.discogs.com/release/15771199-Peace/images/update
Now notice the dimensions of the artwork.
If you click on it, it just gives you a small 600x600 version.
Instead, Middle Click on the VIEW button (or Right click and Open in New Tab - depending on browser)
Now look at that new tab. Big images ready to save.
Even better, ask @st3v3p and I predict he will just say “Right click an image, open in new tab” and it will work from anywhere in Discogs and not just the edit page.
The key is - get an image to open in a new window and the addon swaps it to the biggest available. (And not just from Discogs)