Select the tracks, right-click, Cover Art > Get from Internet...
There won't be any choices if there is only one submission of artwork available for the album.
If you want different artwork, find it on the internet using either MC's links (such as Images) or your own searching (e.g. google images) and copy the link to the more preferable artwork and in MC with the tracks selected, right-click, Cover Art > Paste from Clipboard.