To view album artwork in your library, click the blue 'Artwork' button located to the left of 'Analyze Files'. This will show the 'Art' column in your crates, playlists, and the Prepare panel, displaying album covers for tracks with embedded artwork and a default image for those without. 

You can resize the 'Art' column by clicking and dragging this column left to make the artwork smaller or right to make the artwork larger.

Changing the album artwork of the track will require the use of 3rd party tag editing software i.e. Tag Editor Lite or Kid3.