Increase album art image size limit
You can set a limit for the size of images that will be downloaded. The default is 3MB.

If your images are larger you can increase the limit from:
Settings → Album Art → Max size of album art file

Clear album art cache
If you've made changes to album art images (added or replaced) the album art cache may need to be refreshed. See Album Art Cache
Check location of album tracks

The path to the album art image (e.g. cover.jpg) is derived from the path of the first track with the album tag.

If the tracks with the same album tag are not all in the same directory then the wrong image may be downloaded.

If there are multiple albums with identical name select the Albums by album artist" mode to resolve the ambiguity.