When you set to full size the display of covers, be it with cover display or shelf, the image of the cover is blurry. Look like the thumbnail generated from the original cover is too small.
Would be great to have a bigger thumbnail or 2, depending the size we do use.
Update:
Added an example: Untitled-1 hosted at ImgBB — ImgBB
As you can see, the thumbnail from the left is blurry while the one from right is clean. This is the same image I do uploaded (high res scan)
I did noticed the same on other Clz application. Seems an issue (or per design) on the thumbnail lib you use