@Florian has introduced a new feature in Mp3tag 3.25c (2024-05-08):
Thank you for this useful feature!
Would it be possible to show a cover in a bigger size if we hovering over it?
Currently, I (with my screen resolution and my glasses) can not see the difference between the various covers.
Other than the Development Build Status post and this topic. I can't find any documentation regarding this feature.
How can I enable thumbnails in query results?
I'm assuming there's an entry in [IndexFormat] for it, but I'm unable to figure out what it is.
Just have a look at the new Discogs.inc and you will see the new entry
[IndexFormat]=...%_coverurl%...
and you need a section to fetch the cover thumbnail and fill the complete path to a cover preview thumbnail at the matching/same position as the above %_coverurl%.
I assume, Florian will add the documentation about this feature as soon as the release will be the official next version.
Got it.
I had tried %cover%, %_cover, %thumb%, %_thumb%, %thumbnail%, etc. but was only getting the URL. Now I'm getting the image file.
Thank you very much.
Thank you for the feedback.
What I learned:
The preview cover should be very small because you have to download every single cover from the source. As this could be dozens of different results, they should not be huge high quality pictures.
I hope that we get some kind of "show me a bigger preview" for selected entries. Currently, the preview cover thumbnail is only about 26x26px in my screen resolution.
You can only see a difference if you would manually zoom it up several hundred percents to get this pixelated version:
I’ve already started working on the hover preview yesterday. It’s a nice suggestion, thanks for the feedback!
Great news! 
Just let me know if you need a tester for an alpha version 
@Florian If you already have this thumbnail option built for the source lists, is it possible to also use the same for the mp3tag file list? I believe there have been several requests in the past that this would satisfy.
Thanks for the suggestion! I've added this larger preview (250x250 pixels) when hovering above cover thumbnails with Mp3tag v3.25d.
Fantastic solution, thank you vey much! 
Now I can see at first sight, which album is the "Deluxe Edition" or the "normal" edition and can easily choose the matching one for my existing tracks:
