Book cover quality
I've tried uploading at 400 x 600 (300 dpi) and 3x the ratio and after pressing 'save changes' they always look blurry. Not so blurry to the point where it's impossible to see the image, but definitely not the 300 dpi resolution that the image started with.
I spent a few minutes going through some of the covers in Recent and even Trending and they all seem to have the same problem. I'm not sure if it's me/my internet or if this is actually something that's pretty common here on RR?
I found a thread that had the same problem, but it was from 2019.
Most sites have a system in place to automatically resize images and make them fit their display size. This is especially true if those images are uploaded by users on a front-end form as opposed to trained designers and content editors.
Without these systems in place, many users would upload massive covers (2000 x 3000px) and the homepage would take several minutes to load. This is why you won't see an increase in quality by uploading a larger image
In addition, many sites will apply some form of compression to these images, which is probably the main cause of the blurring. Again, this is done to decrease page-loading times.
I have noticed RR's compression seems to be a bit more aggressive than other sites though. (For example, if you compare the same cover on RR to one on Amazon or Goodreads, the latter two are usually sharper.) But as long as you're uploading a high-quality image at the right size, there isn't much else you can do.