Maybe not most covers, but some become really ugly after resampling.
Is there a way to upload a cover without having it horribly blurred?
this used to be prettier imgur.com/a/ywqefzK
but red letters never look good in JPEG format: bookcrossing.com/journal/15852501
You can enter the covers in jpg or png format, both the system will accept.
If you click the cover of that book, you'll see the picture becomes more clear. Unfortunately our system always resizes the covers so they would look all more or less the same.