If you use last.fm or listenbrainz, simply enter your username and click "Generate". The track will be your last listened track.
If you want to generate an image for a different song, then ignore the username field, and enter the track title and artist name on the second field, and of course, click "Generate"
To download the image, just right click or hold your finger on the image and click "save as"
Click here to view differences with apple music.